I need advice on what to do with my two guinea pigs cages!?

I've had my guinea pig for a little over a month now, she is just over 2 months old. My brother just got an eight week old from the same breeder yesterday. Our original plan was to keep them separated until the baby got older but then we decided to introduce them and they have been inseparable since. They are both females. Although they were separated last night i was wondering what we should do. They do have separate cages large enough for one guinea pig, but not for two. Should we keep them separate sometimes in their own cages and together other times in a too small cage or should we just go out and get a large enough one for two. My only concerns are that they might become unfriendly to people and that the baby may not get enough food. Also im not sure if we have a large enough space to put a bigger cage. Any advice is appreciated =)