Uniting and temporarily keeping 2 queens?

[Sutty]

I moved a very full, well behaved nuc into a full-sized brood chamber today, with the plan to unite it with a grumpy hive after dispatching its queen.
It took 4 passes through the combs to spot her hiding in a space between comb and bottom bar. I couldn't get at her to catch her so popped a QE over the hive and shook the frame onto it - no sign of Q & not on frame!! I can only assume she dropped back into the hive as I reached for the QE.
By now the bees were running around a lot so I called it a day and closed up.
I briefly considered putting 2 QEs on the box (one flat plastic & one wood wireframed so there is a gap) and uniting them anyway, with a plan to dispatch the unwanted queen when I next found her.
Would this have been likely to be successful or would the presence of 2 queens encourage fighting more than may occur otherwise?
If it works would I need more separation than I describe, eg a super between the two brood chambers?

 

[madasafish]

Park spare Qs in a mini nuc with a cupful of bees and some fondant and a QE on entrance.Far simpler.