Need help on enclosure size

I rescued two bearded dragons who previous owners were letting loose and really didn't even maintain the basic care. I have had them for a month and they have good lighting and general care and working on socialization to me. They have been to the vet and apart from being stunted in growth (they are 12 - 14 months old) and are both only 10 1/2" - 11" long. They have grown about 1/2 inch since I have had them and also gotten bigger in general body size. Vet knows one female and thinks other is female as well but only 95% sure.
So long way to get to my question. Enclosure - currently they have a 48x13x15 enclosure (fish tank this is what I got with them) I want to upgrade and especially get one or two terrariums that have the sliding front doors. From what I read at some point even if both are females they will need there own space. They do not fight at this time and have been together since birth. I want to get a home that they each can grow into to. So the real question what size enclosure should I get - gallons are all fine however, from the ones I have looked at a lot increase Gallons by increasing height. So more true LxWxH would be helpful, and any ideas on best place to buy one (live in Kansas) THanks Geoff

They're best off with their own enclosures just so they have some extra space and don't need to compete for resources which can be stressful. I'd shoot for at least a 4'x2'x2' enclosure. You're better off building one if you can. It's much cheaper to do and you'll be hard pressed to find something that size that isn't custom built. Here is a walkthrough of a popular model. https://beardies.dreamwidth.org/2145.html

If you need to save on space, you could build a stacked version of it.