brandonnash is right -- you can try it for a while and see how far it goes before it shuts down. If you're already pushing it to its limits with your current set up, it doesn't have much more to give. Plus, I think you'll get more volume out of the two subs than the one with an increased amp. Two properly set up subs should give you a 6 dB increase, while you would have to go 4x in power to get the same increase in SPL. That amp doesn't have 4x more to give.
That's just what I'm thinking. If I were you, I would do it. That's just exactly the kind of thing I would do.
Just turn the volume up slowly...