Hi Steve.
It's not the number of sticks that matters. It's the number of memory ranks per channel.
Buildzoid has done a video on this called "Testing XMP settings in dual and single rank".
Because of interleaving between the ranks the memory controller can talk to the 2nd rank, while the 1st one is "busy".
This also has the effect that the more ranks you add per channel frequency starts to matter more and timings affect performace less.