If the old hive swarmed today, chances are there is no new queen in it yet. Usually a swarm will issue before a new queen hatches, but not always, as each hive is different within guidelines. If you keep it like it is for two weeks, you can then check both hives and see if they have queens. The new one will most likely have a queen and she may be laying eggs at that time. If the old hive at that time does not have a queen then you can join the two hives back together again by stacking one on top of the other with a sheet of newspaper between the two. I like to put the one with the queen on top of the queenless half. Poke a hole or two in the newpaper with a pencil near the center. The bees will eat out the newspaper and join up more calmly that way as both halves have a common task of getting rid of the newspaper. They also join well because one half is desperately seeking a queen and the other half will have a queen.
20+ years, raise my own queens, feed when needed. I treat, but have not perfected varroa management yet.