There's certainly nothing wrong with this idea -- it has often been said that two subs are better than one. But before I can give you a real answer, I want to know more about your reason for looking into it.
Do you feel your current system is lacking in any specific area? Depending on what you're after, you might actually be better off going with a pair of M80s than an additional EP600. (They're cheaper, too.) It's important to keep in mind that any bass handled by the EP600 that's below the range that the M60s can reproduce is not localizable. Your ears cannot distinguish stereo below about 80 Hz.
The main reasons to go with two (or more) subwoofers versus one is that you can get smoother bass response throughout the room and/or higher output.
