www. is a subdomain in its' own right. It is now pretty antiquated and can/should be ignored; a CNAME record can be used for those who persist in its' use.
Create 2 new accounts:
subDomain1.mainDomain.com and
subDomain2.mainDomain.com
This gives you independent and isolated domains. These can share the same IP address.
Just remember to add in the appropriate DNS entries.