Child Domain

Hi there,
I have a registered domain (www.a.com) and I have an MX record to my exchange server in local domain (server1.x.local),I want to create a child domain ( b.a.com) and create and MX record for the exchange server in second local domain (server2.y.local.
1- Can I create a child domain on the machine where I have my first domain?
2-How can I create a Child domain?
So please if you can help me, and provide details.
Thanks
Lam