They usually are okay if you add them at the same time, but the larger one was allowed time to get used to being the "man" of the tank...throw an anemone to be territorial, and it only worsens the problem.
Maroon clowns *are* sexually dimorphic, so what happens (typically mind you, not always.) when you put one larger one and one smaller one together at the same time? Since most maroon clowns are born female, the smaller of the two will switch and become a male. The only problem with this is that then you have to wait for the little one to grow up
The easiest way I've found to make a mated pair (and this doesn't work all the time, just every lucky once in a while) is to get two clowns greatly varying in size. (One about twice the size of the other) Introduce them at the same time to a decent sized tank (A 55 at least will make the stress of the transition easier on them both) see how they react to each other, if there is any fighting at *all* (because of their cheek spines, they'll often rip each others' mouths...which kills both of them) remove one of them.
If they'll take to each other, they do look a bit like they're fighting, but if you watch them closely, you'll see that they're checking each other out. They'll chase each other around a bit, and then they'll just swim around together. Then, if you have the proper lighting, add the bubbletip anemone. (I've heard they'll take to others, but it's rare, and a whole lot easier to just go with what works in nature.)
The reason you want such a drastic size difference is to not unnerve the larger one. They don't have to be afraid that the little one will take over because it basically knows that it could rip it apart if it wanted to. The reason you add the anemone afterwards is to not give them something to fight about initially. Clownfish are very territorial over anemones. Not just a little, a whole lot...I've seen my tomato take on various lionfish, a marine betta and numerous crabs over her anemone.
Just my experience (But it's a pretty successful amount of experience since I currently own a mated pair of tomato's and a friend of mine has the gold bar maroons)
