Like Dak said, garters are about the only exception I'm aware of where cohabiting is not only acceptable but encouraged, especially when young as accounts say they feed more readily with company and overall are an unusually gregarious type by snake standards.
Anything else, let alone mix and matching entirely different species, is putting potential undue stress to both/all parties at best to setting a timer till one of them gets just a little too hungry/territorial at worst...