My two hives have really grown now, and I need some advice. Do they ever swarm late in the summer or early fall? How big do they get before feeling crowded? Or does feeling crowded have more to do with space (extra honey supers)?
Swarming is bee's natural habit to reproduce. It is nothing to do with size of hive.
Just put honey supers as much as it needs. Put always empty super between broods and honey.
I just installe " ready to get honey"-condtion my hives and I put together hives so that they have 5-6 supers. The biggest have 5 lanstroth + 2 farrar.
It is just now a good honey day. They will catch honey 5-10 kg/hive today from turnip ripe. Distance is 100 m to the field.
let them alone and just add honey supers?
That is right! If you see queen cells, then you change your methods.
split them both and buy queens?
NO, good heavens!
I'd like more hives, so I'd love to start another before winter, but I don't know if it would make them all too weak.
Buy a queen, take a brood frame where it is coming out new bees. Put there one frame of pollen and one empty. So you have start to new hive.
When frame is full of eggs, put larvas to big hive and let them raise them. Give one brood frame again and that little nuc will manage with it's own.
When your honey season is at the end, you can take more brood frames from big nest and you get one box full of bees.
Hive #1 hasn't started on it's first honey super yet, and hive #2 is nearly done with it's first honey super. Both honey supers have top bars.
HOW BIG are your hives now?
In my opinion and experience 3- 4 super hive is not good to gather honey. It need to bee 5-7. 8 and 9 is difficult to handle.