If your hair stylist has an assistant that ends up doing most of the work (wash, blowdry, comb out, etc.), are you expected to tip both?
Not at all, you should tip the one who served you. If they served you both, reserve the tip for the one who did most of the work, in this case the assistant.
I would hand the assistant the tip. If that's the person doing the work then that's person who deserves the tip.
Yes. If given to one, inquire if they split the tips, some places do.
Yes. I give each a tip.