Your use of the word "pointing" is ambiguous.
When adding a domain or subdomain as fully hosted, you get to specify what DH calls the Web directory or what the Apache documentation calls the document root. This is the directory where Apache starts looking for files to serve from.
You can certainly add both domains and specify the Web directories as you see fit, for example you can specify /home/username/domain-a.com/ and for domain A and then specify /home/username/domain-a.com/subdir/ for domain B; this would make http://domain-a.com/subdir/ and http://domain-b.com/ bring up the exact same files.
There is also the ability to have Apache issue a redirect, which is the term for telling the web browser to ask for a different URL - thus the visitor sees the browser location bar change from http://domain-b.com/ to http://domain-a.com/subdir/
Customer since 2000 openvein.org