Question:
How do I host another domain into a single server if it has a different DNS server than mine? I have my DNS from a local ISP; the other is in register.com.

Answer:
It shouldn’t matter how many DNS servers you have. Just point them all at the IP address for your Exchange Server (or point of entry, like firewall or proxy) and set up the appropriate SMTP addresses on the server as if there were a single DNS server.

Exchange doesn’t care how the messages arrive at its doorstep; it simply accepts the ones intended for it and bounces the ones that aren’t. If you have the SMTP addresses for both domains configured properly, it’ll accept the mail for them.