In order to re-pair birds you would have to isolate them from the rest of your birds. Stick them in a nest box with a divider so that they can't get to each other but can see each other. I would say to do this for about a week or until they get to know each other then take out the divider. They should pair up after that. If you let them back in with the rest of your birds to early they might return to their old mate. You have to use your judgement on when to return them with the rest of the birds.