Appropriate water changes is what is going to be the most important issue.
I am a university student going to be moving into a dorm and I have 3 betta's I have to take with me. Not much space and they are all in 1 gallons, so I feel your pain wanting to get more, but not having the space lol.
You can get a small 5 gallon rectangle and divide that into 2 for 2.5 gallons each, or if you really need to get a 3 gallon and split it into 2 giving them 1.5 gallons each.