Let's clear up a few misconceptions here.
You don't have a 2.1 system. You have a 2 channel sub/sat system with a shared bass module. If you were to add another one just like that one, you will still have a 2 channel system, but now it will be running another 2 channel sub/sat system in parallel.
Ultilmately, you still have a 2 channel system. If that's ok with you, then simply hook up another set in parallel if your sound cardcan handle it.