That sounds like an odd size. Is is 30" tall, 24" long/wide and 15" deep? How many floors does it have? Ledges?
A kit is
usually accepted better than an adult with another adult,
but this is not always true ... should the older one decide it does not like the new kit ... you will probably find it dead in the cage when you come home one day.
It is best to get the two at the same time, but you have one already, so that is not an option, now.
I would strongly suggest getting a second cage, possibly the same size and style. Should they decide they want to get along and be with each other, it would not be that hard to convert the two into one cage.
Should you decide to get a large cage instead, I would keep the first cage, just in case you need it and I would only allow them to be together when you are with them and awake, just to be safe for a while.
Should you get a large cage and not have room for both, I would make sure the large cage has a solid divider that can be used to keep them separated, if needed. The cage wire alone can not protect one from the other completely. They have been known to bite noses, toes, tails and ears through the wire. The separation would have to be at least 3" apart OR separated by a solid wall they can not chew through, should they not get along with each other.
Hope this helps!
Jo Ann