When confronted with this specific scenario - wanting more gas to extend your bottom time - the best choice is to go to doubles.I’m in the position of being on the verge of wanting to extend my dives a little, so I’m going to have to get a second set of regs and tweak my hose layouts a little.
I’m trying to work out the best thing to do, straight to twins? Get a stage/pony of an appropriate size to be used later?
In reality, while I can appreciate the issue of space, the need for a reasonable configuration really supersedes considerations of what you can 'sneak into' your living space. A set of double 80s is a good choice, probably better than a backmount single, with a pony bottle. Go ahead and move to doubles now, LEARN TO USE THEM PROPERLY. and you will be well served.