If you combine two 8ohm cabs that will give you a 4 ohm load. Your amp is rated 500 watts @ 4 ohms so that means that each cab in that situation would be sharing the available capacity of 500 watts... so technically the most that each cab should be getting is about 250watts at full output on the amp.
I've personally always gone the route of a higher powered amp knowing that I just cannot push it to max. A few years ago, when I could lift more, I had a rig with a separate pre-amp and power amp, the power amp's real output was 700watts each channel at 4ohms. I had connected to it two 4 ohm cabs that could handle 400 watts each. I technically had way too much power for the cabs but never blew anything, but that setup made so much volume that I'd probably be completely deaf if I were to get to push it to full power. I went this route so that I wouldn't have to push the amp as hard in louder volume situations. A few years and a shoulder surgery later and I would say that it was a bit overkill... small light things attract me now over raw power.
