I’ve just started experimenting with the Marvels along with iPhone and Apple Watch. There’s no question that both can be (and stay) paired at the same time. The issue has to do with switching between the two devices; what consistently works for me (ok, twice) is simply 1) power off/on the hearing aids and 2) connect to whichever device I want (i.e. iPhone or Watch). Both sound great with music, by the way; quite impressive so far.
More experiments tomorrow… (Neville; what devices are you using? Can you try what’s worked for me?)