Size Size advice please

I recently adopted Twiddle, a 2.5 yr old little girl cavy, but did not know they were social animals and wasn't prepared for a buddy. I relied on advice from a friend who has had cavies throughout his life, but probably not the most knowledgeable. He doesn't even know about a c&c cage, and thought the petstore cage was fine... I would like to give them plenty of room, but living in a studio apartment I am limited on space. I was really hoping that a 2x3.5 cage would work, maybe if I add a second level. I really want to give them all the love and care they could possibly get. Any advice will be greatly appreciated.