To do this, you would just need a "Y" adapter to split your RCA output into two signals. Connect subwoofer #1 to one side of the "Y" adapter, and subwoofer #2 to the other side of the adapter.
You would only need a separate amplifier if the 2nd subwoofer is a passive subwoofer and didn't include any amplification.
To ease the process of going to a 7.2 system, you might consider buying the same subwoofer for #2 as you already have for #1.