90% of what I've read says to add one that's significantly smaller. Maroons from what I understand are the hardest clowns to pair. I know there's a good article somewhere. Check out the stickies in this forum. I'm having my LFS hold a juvenile maroon for me, so I'm going to be trying the same thing next week. Here's a synopsis of what I read says about pairing maroons:
Like I said, the second maroon should be significantly smaller. I can't remember how much smaller, but I'd say about half the size of the one you have now. That's to make it less likely that the newcomer will submit to the established clown sooner. Next, you'll want to keep the two fish separated, but in a way they can see each other. There are probably a couple ways you can do this. If you can put your quarantine tank up next to your main tank, that may work and have the added bonus of getting the two fish used to each other and accomplishing the quarantine in one fell swoop. My plan is to put the new guy in a hang in breeder tank I made for just the occasion. It'll be pretty close to the female's anemone, so they'll get plenty of exposure to each other. After a couple weeks of this, release the smaller fish into the main tank. You can probably expect some fighting still, but if the smaller fish starts getting hurt too badly, put him back in the breeder/QT. I couldn't tell you for certain exactly how badly is too badly, but from what I've heard, you can expect some minor fin damage. These guys heal pretty quickly. If all goes well, great. If not, try it a couple more times, separating them for another 1-2 weeks each time. If they're still not getting along, you're probably out of luck.
Hope that helps a little. I'm sure there are plenty of people with actual experience who can talk about what they did. There are all sorts of different methods, but this one sounds to me like it would have the greatest chance of success.