I have a website which has two domains added. Both domains point to the root of the website. Is it possible to alter the robots.txt so that one of the domains doesn't get crawled, while the other still does?
You ought to perform a 301 redirect to your preferred domain, which tells robots what URL you want crawled.
For example
www.domain1.com
Should 301 redirect to
www.domain2.com
Similarly
www.domain1.com/Contact-Us/
Should 301 redirect to
www.domain2.com/Contact-Us/