If Q can fly, she should have a fully-developed thorax (where the flight muscles are).
It is the size of the thorax that prevents her passing through a QX.
The idea presented was to let the 2 Qs fight it out. The bees may fight as well. Chucking three or more colonies/swarms in together (plus icing sugar, flour, smoke, room freshener, etc ... take your choice or use none) is supposed to produce such confusion that the bees don't fight. But surplus Qs will. Or else depart with some bees - hence the QX underneath.
Personally, if there are two nucs available, I'd box them separately and retain all the resources for examination and exploitation. If its one swarm that has split, they would be expected to choose to combine themselves when they get the chance. Otherwise one can then take some time to decide the best way forward in the light of your own particular circumstances.