You are opening up a can of worms here. Be very careful trying to use one mixer and two power amps. Some amps need a proper load to work properly and safely, without damaging the amp. And you CAN damage your speakers, if they are rated for 300 watts, and your putting 500 into them.
Also be aware that different companies have different ways of determining watts. For example Yamaha is known for underrating their power amps and Peavey often overrates theirs. I don't know all the details, but I do know you must be careful. Let your ears decide.
Probably you could use one amp for mains and the other for monitors, or power two sets of speakers. I definitely wouldn't try to power one set of speakers with two power amps unless you really know what you are doing.
DonM