Well seeing as alot of people keep them in breeding racks and they live in burrows, not much. So if your doing vivs, you probably want it arounf 15" at least so water bowls and hides can be removed easily.
3' long by 18" deep for one snake. I wouldnt reccomend keeping two in the same viv, it can cause a few problems. But if you wanted to breed them, you wouldnt need to put them both in a bigger viv or anything like that.
If you decide to keep 2 in the same viv (I highly recommend not doing so) then yes you will need a lot more space with many more hides and places for them to be apart when they want to be.