There is also quite a bit of evidence around now that says the bees will ensure both queens don't damage the other if they emerge at the same time, the warder bees will keep at least one confined so the other can deal with her or 'rig' the fight so their chosen queen wins.
That seems to be the way that casts can happen at all.
Q1 would like to deal with Q2 etc ASAP.
Only if she is prevented (by the workers) from doing her instinctive priority job of killing all rivals, can
any cast ever issue forth.
The more workers, the easier they can protect other Q(s). Eventually after lots of casts there are too few workers left to protect any remaining QCs, and casting stops.
However,
the closer together the only QCs are within the hive,
the easier it is for Q1 to locate and destroy Q2, and the harder it is for workers to keep her away from Q2 in another part of the brood area.
Hence, if choosing to leave two QCs to double the chances of one emerging, it makes great sense, to me, to ensure that they are as close together as possible.
Incidentally, I don't worry overmuch about the possibility of a 'winner' emerging damaged. After all, this Q/Q struggle is part of the wholly natural behaviour. It is something Qs are supposed to do!