I'm learning just now that you can even customize both of them. I never thought to do both, just whichever I feel like playing as for that playthrough.
You customize both but "later" is a stretch. You will never see the other after bout 10 minutes of gameplay. >!Spoilers!<. I think they use both of your looks to generate your child but honestly that side story is such shit - I'd rather not look for my son/daughter.
Nonsense. If you ever want to >!go back to the vault, you can see their frozen body slumped over next to your open cryopod.!< That's like a whole 12 minutes!