Betta's can be kept in small volumes of water but it isn't advisible.
The water gets polluted quickly and the fish hasn't enough room to really swim / investigate.
It's more difficult to keep a betta healty in a small tank than it is in a big one.
Betta's that are kept in larger tanks live longer and don't get sick quickly.
If I read the posts on this forum I see many posts of people with sick betta's....
Room, excersise, frequent waterchanges, lots of plants and good food are very important to give your betta a happy and long life.
Some times no matter how well you take care of your fish the will get sick but many fish deadths could be prevented with proper care.
I would just keep one betta in that 5 gallon tank.