There may be something in this.  I read that a proportion of Varroa mites are
sterile or do not reproduce successfully for other reasons.  This may be
among the reasons that they do not increase as rapidly as they might achieve
in theory.
It is to be expected that out crossing would help with viability.  On the
other hand it is extremely likely that any two adult mites in a hive are
already very closely related so two adults in one cell may not necessarily
result in an out cross.  This is most likely to occur when there has been an
influx of  mites from another hive through robbing and is one more reason for
the beekeeper to prevent robbing.
If you get too many adult mites breeding in one worker cell are they likely
to kill their host larva and thus fail to reproduce at all?
Chris Slade