IMO, and a number of leaders in the field, it is bettter to add one that is considerably smaller than the original.
I have a little screen specimen box(about 5"x4"x4"), that I put the smaller clown into, then just capped it and placed it as close the the BTA as I could so the Bigger one would get use to it. I left it there with the lights off for about 4 hours and then released the smaller one. He went to the BTA and the Larger one took right after him and he swam away. THe littler one just kept swimming around and the bigger one finally after about 2 days accepted him and now they are a mated pair. I didnt re-arrange any rock although I was told that it isa good idea to do it so that it makes the origianl clown have to find new territory, but I had a BTA so didnt matter where I moved stuff she was going back tothe BTA.
Reason why to get a smaller one is...if one clownfish is in an enviroment it will become a female, if you place a bigger one in there you may end up with 2 females anf then 1 dead fish. By placing a Considerable smaller one in you have a VERY higher chance of mating as the smaller one might not have started to change to a female.
GOOD LUCK
Below is a picture of the female claiming dominace.