I've housed adult tigers together with no problems. I would not want to use anything smaller than a 20gal long for two adults. One time, however, I had two adults together (same size) - one was docile and one was quite aggressive. The more aggressive one was always stepping over and stepping on the more docile one - this resulted in the docile one getting some substrate in his eye (from being stepped on) which caused a mild ocular abrasion which was treated with sterile ophthalmic Neosporin ointment and sterile amphibian ringers solution as a wash. After that, these two were separated.
As Kaysie says - if there is a significant size difference, this could be an issue...they are predatory carnivores.
