I'll try to explain a bit better since I obviously lost you somewhere...
Gmail is very different from froogle, but both are ran by the same company.
Your two sites are very different from eachother, but both are ran by you.
The google example is a good one - gmail.google.com, froogle.google.com. Completely separate so files between the two don't interact or affect the other, but both on google.com.
Now replace gmail and froogle with your subdomains (for the sake of explaining, call them commercial and personal), and google.com with yourusername.x10hosting.com. They'd be separate but both ran by you. It'd look like
http://commercial.yourusername.x10hosting.com, and
http://personal.yourusername.x10hosting.com.
If that doesn't quite do what you're looking for, then you're either a) going to need to use different hosting since you can't have more than one free account, or b) going to have to pay to have one of the accounts on x10.
Personally I don't see the problem - my old host had 2 subdomains in addition to the main one, and the only way to tell that they were all mine (aside from the copyright thing at the bottom) was to notice the url bar had the same ending to it. Can you explain why the subdomains wouldn't work?