Using one single top bar will solve the issue of having the divison board moveable; this is something which I would think is very desirale as it's not only good for uniting pre-winter but (perhaps more importantly) it offers a quick solution when one side fails to get a queen mated. Much better in my oppinion to unite to the queenright side and then divide again when you're distributing the next round of cells. Averaged over the season I don't believe there's anything to be gained by pushing on, giving new cells to mating nucs which for one reason or another have already failed. Re-unite and give them heart by being part of a queenright family. It does neither side any harm.
As for that vent in the floor, fill it in
. Irrespective of whether you think mating nucs should have floor vents (I don't) I can confidently say that in this case the mesh is definitely in the wrong place (both sides next to the division). It will complicate management of a twin unit....
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Although I've been a big advocate of the miniplus hives (and still like them a lot) I reckon that you're going the right way using standard frames for mating nucs but that's another thread!!
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