If you leave 2 Queen cells in the queenless colony, does the first Queen to hatch leave with a cast straight away or does she need to mature further before doing so?
If she does need to mature further could a factor in whether you lose a cast, or the new Queen kills the remaining Queen cells be the different stages of development the remaining QC(s) is(are) at?

This year I thought I had removed all but 1 Queen cell as part of an AS (the queen was definitely in the other section), however on the following inspection (I must have timed it well) there was a cast in tree. When I went through the hive, I found a Queen cell 'not' where I expected, and in the one I had intended to leave, the Queen hatched not long after.