if you have several partitions configure in devfs, them all are
updated. If you config the mirror before dding, then dd takes
care of everything not updated during the dd, and devfs takes
care of things updated while dding. Since devfs services reads
from the first device (as long as it works), there's no problem
in using it.

There's a race of course, but you can probably check later with cmp
(I did that a couple of times and the race did not hurt me). I'd try
as hard as feasible to keep the partitions untouched, just in case...