Can't agree with Aimless on this more. Cohabbing of any nature outside of breeding purposes is already controversial and risky business- it really all comes down to the particular keeper's views. It's generally accepted that F/F pairs can be housed together, given an appropriately sized enclosure, but this still is not concrete and females can still fight. M/F pairs WILL breed. If they don't, you probably aren't keeping them in proper conditions anyway! But if you don't wish to be a breeder at this point, just don't house F/M.
Most importantly, make sure they are similar size. Even F/F pairs will become aggressive if there is a large size difference.
My opinions on the matter? I come from a snake keeper community, so my fundamentals are no cohabbing with reptiles, ever. I do not own garter snakes, and colubrids and ciliatus are not social by nature. They do not require companionship, and I believe they live better quality, stress-free lives when housed alone.
Just remember, if you're thinking about cohabbing, "It's never a problem, until it's a problem."