If they're from a pet shop they're probably hybrids (if they look like campbells or winter whites but you don't know their family history they're virtually guaranteed to be hybrids to some degree or another). Theres nothing wrong with hybrids though, I have two at the moment and had four until a few weeks ago
I'm glad they seem to be getting along well so far, I hope they continue to do so. Good luck with the ideas for a new cage and I'm glad you have a backup aquarium incase they need separating
If you mean a separate burrowing area as in something attatched to the cage it may be a tad risky, but a burrowing area in the cage or just deeper substrate in the cage could be great - our hybrid girls have 6" of substrate and if we need to wake them its often a bit like a giant game of hide and seek figuring out where they've buried themselves this time lol