Premature breeding if they're unsexed/sexed incorrectly is the biggest concern, as kcaven said, cases of cannibalism are very rare.
As Bram said, they're not social creatures and living together in a confined space would stress them out. Also, this new snake could harbour parasites, mites etc. that could transfer to your other snake, as you wouldn't be able to perform any type of quarantine.
All the best
David