Are the 2 colonies approximately the same age, strength and size? Could just be a hatching out of new bees on their orientation flights. Wax bits below is normal 'if' your hives have SBB
Robbing 'looks' very nasty, lots of "obvious" fighting, chewing, chasing, noise and many DEAD BEES.
Seems there's alot of confusion over robbing behavior this season. Fed hives can and will rob another usually weaker hive, even well fed ones if they have room for their loot. Where are you placing the syrup? Inside hives or outside? If outside, move feeder away at least 100 yards (what's metric for a yard, meter right?)
If you're certain of robbing, close the victim entrance (s) down to ONE BEE size. That's step one, but I'm not yet convinced until some questions are answered