Administering multiple domains

Administering multiple domains

I own two domains on different topics, and administer a third domain which my son owns.

1. Can I set up separate accounts for each of my domains, or do they both HAVE TO BE be setup under one account?

2. I need to set and administer an account for the third domain that my son owns, but administer it on my computer and have both accounts open at the same time. Is this possible?