2 multisites using subdomain not folders, on a single hosting acct?

I have a greengeeks hosting account, which is all you can eat in terms of space, bandwidth, subdomains, addon domains, etc. (shared server). I set up my first wpmu and have been creating subdomain sites on it quite successfully. I tried creating an addon domain and then installing wpmu with subdomains again but they don't seem to be resolving.

Am I wrong to think that I can have 2 domains with wpmu on a single domain hosting account?

Thanks!
John