I learned the hard way that maroon clown fish are among the nastiest. A guy at my LFS said you have maybe a 10% chance of successfully pairing one if it hasn't been paired already. You may help the odds in your favor by ensuring that the new one is much smaller than the existing one. Your existing one is most likely female by now if it wasn't already, and is therefore more aggressive. A smaller one may not threaten it as much. My LFS has a pair of peaceful maroons in a display tank, but there is a huge size difference between the two.
Like you, I decided to chance it anyway and I brought home a second one (non-maroon). I asked my LFS how long it would take before I knew whether they could co-exist, and he said "oh you'll know immediately. If they don't get along, one will most likely be dead by the next day."
As a result, I made sure to be prepared to isolate them if necessary before introducing the new one (see below for description of isolation chamber). The existing maroon took no interest during the acclimation process, but once the new guy was out of the bag, it took only about 20 minutes before the maroon was bashing into the glass trying to ram the new guy. It was unsettling to watch, the crack of the fish's skull against the glass was terrible. They can actually kill themselves this way so it was imperative that I separate them immediately. Fortunately I was prepared and isolated the new guy right away and took him back to the LFS the next day. When I told them why I was bringing him back, they inspected him very carefully before agreeing to refund me. If I had let the abuse continue, I would not have been able to return the new clown.
The point being, if you're going to give it a try, be prepared to observe the introduction for a few hours and be ready to get the new addition isolated quickly if necessary (assuming you want your money back).
As an aside, I discovered during a previous emergency that the plastic mesh bowl inside my wife's salad spinner makes a great isolation chamber within the tank. Of course, once she saw me use it in the tank, she doesn't spin salad in it anymore, but it works perfectly!! It floats just enough that the upper edge is a millimeter or so above the water line, leaving the entire bowl benath the surface of the water, acting like a cage. The plastic grid-like mesh allows secretions to exit and freshly oxygenated/filtered water to circulate.
