It really depends a lot on the two cats. I seem to have had the best luck with 2 neutered males living happily together, they get along very well, groom each other, play together, etc.
Bad luck for me w/male & female, or 2 females (all of them "fixed"). Retaliatory pooping and peeing (mostly pee), spraying, bad attitudes all 'round

This went on sporadically for several years and even over a move into a new house, so I don't think it was just a matter of time and getting adjusted to each other.