I've only had 1 UVB in there (T-5 reptisun tube) but it of course only fills one side with the UVB rays & the other side of the tank is kind of dark and he doesn't go over there much. I've been planning to add a second UVB, same kind, to the other side of his tank to brighten it up but would that be too much UVB rays?
How long is your current tube?
And how tall is your enclosure.
For me to get 0 uvi anywhere in my 4x2x2s, I have to use a 22 inch t5. So for me a 22'' is best for my 4 foot long tank.
I'd imagine that the 34" t5 would be sufficient for your 5 foot tank. You want their to be an area in the tank with little uv and no uv. A gradient just like with heat.
You can always use a non uvb bulb to add extra lighting to a tank. That's what I'd do in your situation.