500w RMS would be ample for just about anything you'd want to drive. i wouldnt expect a massive difference from a second subwoofer though. the general rule is, 2 subs are 3db louder then 1 sub both subs being equal, and im assuming the 308 isnt that much different to the 307.
the question is, why do you feel you need two? are you finding it isnt loud enough? i had a little 150w rms vibe amp powering my 12" subwoofer initially, and it was LOUD although it did struggle with very low frequency stuff. since changing that amp to one capable of 500w rms @ 2 ohms, it's terrifically loud. I'm talking louder than a night club loud and it really slaps that subwoofer about yet the amp is barely getting warm.... but im digressing. anyway, the point is...(and there is one, trust me lol).....if it's not loud enough, you'd probably get a better result changing the current subwoofer for a 15" than you would buying another sub of similar specs. But, regardless of that, it should still be more than loud enough if you've got that amp wired up in bridged powering the one subwoofer.