Okay, what Reptisun 10.0 UVB tube did you purchase, the T8 or the T5HO? This is easy to accomplish, I think you are just thinking too hard about it...I believe you either have a 40 gallon breeder tank or a 20 gallon long tank if I remember correctly, so either way this is not that difficult. Both lights need to be located in the Hot Side of the tank and both need to be directly over whatever his main Basking Spot/Platform is. When I say "directly over" obviously you can't put both lights right on top of each other, but you can still get them right next to each other and over his Basking Platform.
The reason I asked which strength of tube you bought is to determine how close the UVB tube must be to the Basking Platform, the T8 UVB tube must be within 6" of the Basking Platform, while the T5HO must be within 11" of the Basking Platform. The T8 UVB absolutely must be mounted either inside the tank and underneath the lid, either by cutting out the mesh from the lid directly underneath the UVB tube or by simply cutting 4 small holes in the mesh and then using either those really long, plastic Zip Ties, Twine, Shoelaces, etc. to go through the holes and around the entire tube fixture, strapping it and tying it to the underside of the mesh lid. This is absolutely the best and most effective way to mount the fixture inside the tank instead of "Cutting out the mesh from underneath the UVB tube while it's sitting on top of the mesh lid", because if you do it this way as you suggest, the
UVB light will only be emitted directly underneath where the UVB tube sits, because the metal reflector that is behind the UVB tube inside the fixture will not be able to reflect any
UVB light throughout the tank (Unless you cut out enough of the mesh in the lid so that only the very edges of the fixture on both long sides of it are all that is sitting on the mesh lid and the entire UVB tube and all of the space going down each long side of the fixture is unobstructed by mesh too, and even this is not going to get enough of the
UVB light reflected throughout the tank. So you really are best to simply strap the entire fixture to the underside of the mesh lid using either the long Zip Ties, Twine, String, Shoelaces, etc. This is how I have my 22" Reptisun 10.0 T5HO UVB tube and it's 24" tube fixture mounted inside my tank.
I still have my tube fixture strapped to the underside of the mesh lid in my very large glass Exo Terra even though it is the 10.0 T5HO. I just feel better having the
UVB light unobstructed by anything (I have 3 dragons, 2 of them are in huge custom, wooden enclosure where the UVb fixture is built-in to the ceiling of the enclosures, and my rescue guy is in a very large glass Exo Terra with a mesh lid that has a plastic "grid" on it's top that separates the top into 4 squares of mesh). I don't know what type of tank you have or mesh lid you have, but it really doesn't matter, strapping the long tube fixture to the underside of the mesh lid is going to be the easiest and most effective way to get the UVb tube fixture inside the tank. I don't know how tall your enclosure is either, but as long as the top of his Basking Platform/Spot that is located on the Hot Side of the enclosure is within at least 11" of the unobstructed UVB tube/fixture that is strapped to the underside of the mesh lid, the distance will be fine.
As far as setting up the lights, this is going to depend on 2 things: The first is what type of fixture you have his bright white basking bulb that is over his Hot Side in, meaning is it a Dome-style fixutre that must sit directly on top of the mesh or is it a Clamp-style fixture that clips to the edge of the enclosure, and the second is what type of mesh lid it is that you have, meaning is it just a big, flat, solid mesh lid, or does it have a plastic grid on top of the mesh like I described mine has. These are going to determine how you have to have your Basking Light mounted and where you will have to strap the UVb fixture to the underside of the mesh lid. Also, what you use as his Hot Side Basking Spot/Platform is going to be a factor.
We don't want to make this complicated because it's not complicated, so basically all you have to do is strap the UVB tube fixture to the underside of the mesh that is over the Hot Side of the tank and not right against the back or the front walls of the tank, because you don't want to have half of the
UVB light being reflected directly into the glass wall and lost. I don't know what length of tube/fixture you got, but you want the UVB tube to cover his Basking Platform on the Hot Side of the tank (completely cover the Basking Spot/Platform on the Hot Side) and then any extra tube that sticks out on either side of the Basking Platform is fine, it doesn't matter as long as his Basking Spot/Platform is completely spanned by the UVB tube.
So you may need to move his Basking Platform a bit if it's against the front or back wall or the side wall of the Hot Side, you want it more in the middle of the Hot Side so that the UVB tube is not right against any of the glass sides of the tank, and is at least a couple of inches away from the sides. Obviously you'll need to leave enough room on top of the mesh lid on one of the sides of the UVB tube for the Basking Light Fixture to either sit right on top of the mesh in a Dome Fixture, or so that it has enough room right alongside the UVB tube to shine down onto the Basking Platform if you have it in a Clamp Lamp.
******* I'm assuming you have a Dome Fixture that sits on top of the mesh lid, so what I would do is just basically sit the Dome fixture down on top of the mesh lid over the Hot Side of the tank and make it be the light that is against either the front or back of the tank (put the Dome Fixture against the front or back glass of the tank), and then you'll simply strap the UVB fixture to the underside of the mesh lid (the UVB tube will be towards the center of the lid because the Basking Light Dome Fixture will be the light on the OUTSIDE of the lid). So one side of the dome fixture will be next to the outside of the mesh lid and next to the front glass wall or the back glass wall, whichever you want, and the other side of the dome fixture will be on the inside of the lid, towards the center. The UVB tube fixture will be strapped to the underside of the lid starting right next to that inside edge of the Dome fixture...So once you put the Dome fixture on the top of the mesh lid, over the Hot Side and against either the Front or the Back edge of the lid, then poke 2 small holes in the mesh lid right alongside of the inside edge of the Dome Fixture. You'll want one hole for the Zip Tie or the Shoelace, etc. that will go around one end of the UVB fixture, and the other hole for the Shoelace etc. that will be around the other end of the tube fixture.
I just set my dome fixture down on top of the mesh lid, over the Hot Side and right against the front edge of the tank, then I set the UVB tube fixture on top of the mesh lid right alongside the inner edge of the Dome Fixture, allowing maybe a 1/4" gap between the two fixtures. then I poked 4 holes in the mesh, basically 1 next to each corner of the UVB tube fixture, but in towards the center of the tube fixture so that the Shoelaces or Zip ties would be able to wrap around each end of the tube fixture. So the Basking Light sits on top of the mesh lid right at the front of the tank on the Hot Side, and the UVB tube is strapped to the underside of mesh lid, over the Hot Side and towards the center of the Hot Side of the tank and right alongside the Basking Light.
Then just remove the mesh lid and put it down on the floor upside down, and thread the Zip Ties or whatever you're using to strap the tube fixture to the underside of the mesh lid through the holes, then just set the tube fixture with the UVb tube installed inside it already on the underside of the mesh lid and lined up with the holes, and wrap the shoelaces or zip ties around each end of the tube fixture and lock the zip ties or tie the laces/twine, and then cut off any excess of either that is hanging. Then just put the lid back on the tank and set the Dome Fixture at the front of the tank right in front of the tube fixture. Lastly, then you need to move the Basking Spot/Platform so that it's centered in the middle of the 2 lights. Done....
My UVB tube fixture stretches across from the very end of the Hot Side of the tank, all the way across the Hot Side of the tank, and then a portion of it goes into the Cool Side of the tank as well. All you have to be sure of is that you put his Basking Platform centered under both lights so that the place he sits/lays while he's basking is completely covered/underneath both lights...