From my personal experience I can tell the bigger one will beat up on the smaller one. So I had a pair of clown for about couple of months when all of a sudden the Darwin died (still don't know what happened). I caught the other one and put in a 10g tank so I could observe and make sure this one survived and nothing was wrong. After observing the fish for another few weeks, I decided to add a small Darwin clown. Within seconds of adding the new one, the older-bigger one had the new fish pinned down on the glass floor. You have no idea how freaked out I was. I took the net and separated them while catching the smaller one. One interesting fact, while I was freaking out, I realized the new one was doing the "shake", even while it was almost pinned down. Since I did not want to risk killing it, and I did not have an acclimation box. I had a plastic Walmart pet boxes that had a cover, I drilled few holes in that box, filled it with water and let it half sync in the tank. The little one was always following the older one from within the box. The older one tried attacking the box for few days. Things calm down after 3-4 days. I waited another week, then released the smaller one with the older one. The older one chased the little one for few minutes. I was freaking out, but I let them sort it out. After about 5 ~ 1o minutes, they seem to be getting along. That was over 2 years ago, and they are a mated pair now. It's doable, just be cautious and patient.
