I am no expert at this, and please wait for a voice with more experience to speak up, but. I have always seen a set neck guitar neck glued into place before finishing. If you did them separately, too much of a chance of getting finish where the glue joint will be, plus gluing and clamping on a fresh finish doesn't sound like a good idea to me. As for front and back at the same time, depends on what you are doing, paint, stain, a burst, or clear coat.