If you have another hive, take a tray of open brood and eggs and give it to them. That will keep them busy and entice them to stay. Works just about all the time.
Another thing I do is put feed on for them (sugar water) - especially when I don't want to open another hive right then. They get busy sucking it up, and start drawing comb - however this might also encourage re-swarming, so be careful on this one.
As for 2 clusters on the outside - I've seen this when I knew there was only 1 queen. Not really sure of the cause. It stops once they decide to stay (usually).
The last swarm I caught (2 weeks ago) was getting ready to leave the next morning - all clustered up on the outside. I dropped the feed jar onto the inner cover and they all ran back inside within 4 mins. I'm guessing the queen started laying soon after in the fresh comb I'd put in. They're still there, all settled down and happy now.