Sorry to necro but it is *technically* possible to have as many player heads for one user as you want. Though you'll have to change your username.
Basically, choose a skin that you want and get the skull. Then change your username and skin. The original head should remain the same. You can then get a new skin and use /skull with the new username. You'll effectively have two skulls that were gotten from the same user but different username, this can repeated for every username you choose (limiting you to one skull per month per account) but it's a nice way to get more heads.