forcing them to pair up always a tricky part. I found that sometimes if u give them a little bit of a push, they kind of pair up really fast. Try remove them to a smaller tank or refugium, they will get a little be stressed, but u might found that after a bit they start to hang out together.My guess is when they get scared they will look for similar looking buddy to stay with which they think its the safe way.
BTW, yes u need to choose a really small one then a huge one compare to the small one.