There may be a few who might say I have been lucky, but I have kept alterna together in pairs for 20 years w/out one incident of cannibalism. Alterna, as a rule, are not agressive snake feeders like the getula group.
You do have to take the precautions that Max mentioned previously. I might add that I have found it best to keep only adult sexual or female pairs together to avoid any sexual agression during the spring that can occur if you house males together. I also feel it's best to keep juveniles separate, waiting for the snakes to mature before caging together.
My set up allows for the 2 cage mates to be separated in the same cage during feeding. I will keep them separate for many hours after feeding, if not over night. This allow for the feeding response to subside before allowing them to be together, thus decreasing the chance of a negative outcome.
Good Luck. Now you can have twice as mamy alterna. After all you can't have too many. DB