Assuming you meant that you'll have a seperate 20 long for each snake:
All of the erycine snakes except for the Indian sand boa could probably be alright in a 20 long. Rosies, rubbers, various sand boas, stuff like that.Males of most of these species stay smaller than the females. Sand boas in my experience seem to do well cohabitating with conspecifics in most cases.
There are probably some smaller kings and milks that would do well but I'm not positive on which type. My male hognose is turning two and has yet to outgrow a 10. Depending on your budget, some smaller Asian rats like the mandarins, bamboo racers, rhino rats, ect. would be happy in a 20 long.
I'd just generally stay with things under three feet, and under 2.5 if you want to make sure they have lots and lots of room.