Just looked back through my hive notes…and considering your suggestion of not treating the stronger hive with the lower (4%) mite load. That is also the hive where I noticed a whole bunch of weird looking dead bees out front earlier in the summer. Folks here had speculated that they could be drone that the colony was purging…maybe they ARE a good breed of hygienic bees, and are naturally controlling the mite population.
So I guess I could take a flier and let mother nature know best (any other metaphors I could mix here??) and only treat the hive with the higher mite count. Of course I recognize that my sample size could be off, I should have sampled from more places, etc… But at some point I need to make a decision and stick with it. I am a bit like you in that I prefer not to treat if I don’t have to - treatment-resistance is something that scares the heck out of me, whether it comes to humans or bees!