Tank size?

I currently have two 09 hatchling corns who are very peacefully living together in a 12gal aquarium.

First off let me say that I know both sides of the fence when it comes to the keeping-corns-together debate and I understand the risks of co-housing. They are the same size, they get fed seperately and they have never shown signs of aggression towards each other. I am still keeping an eye on them and can seperate them if need be.

Whew! Now then - My questions:

a. when do I move them to a bigger tank

b. when I do upgrade their tank size - what size should it be?


Thanks!

My personal minimum is the length of the snake should be no longer than the length x width of the cage. You can go much larger as long as you can provide the right heat gradient and provide them plenty of security through hides (the more the better!!) Larger with more hides would also be preferable since you're cohabiting, the snakes should both be able to choose what temperature the want to be and avoid each other if they wish.
I have a pair of 4ft corns in a 50 gal.

Awesome - thanks!