Additional domain in AD

I have an AD controller on windows 2003.
For the longest time, the domain in it has been (example) domain.com
Using (say) ip 10.0.0.0

Now I'd also like to add example.com
Can I create it on the same AD but using ip 11.0.0.0 ?

A couple of follow up questions:

1. There are a number of secondary AD servers to backup the primary one.
1a. Should I assume that example.com will be replicated on all AD Controller?
1b. Should I assume that the two domain need to be on 2 different LAN port on the AD?

2. The two domain clients need to be kept separate. I.e users on domain.com mostly cannot join or access example.com.
Can this be done?
2b. If a user (me...) need to be able to access both domains, can I do it on one account?

3. How can this be done? (detailed how to or link please, I'm not an IT person but have little experience in DIY domain for our own small business.)

Thank you