Are you playing in situations that require more sound than the Fender can produce alone? If so ... you can double your output this way. (and ONLY this way)
Listen carefully
READ carefully?
)
The Fender mixer section has outputs that can drive outboard power amps, as well as the hookup to the internal ones.
You need to determine if either of your amps is capable of running in BRIDGE mode (both channels act as ONE).
If you can't be sure of this - DON'T try anything.
If Bridge mode is possible, then you can run the left channel of the mixer into the Fender amp and send it to ONE speaker, and the right channel to the other amp etc.
NOW - Here's the disclaimer:
Is this really necessary? I had the 8 channel version of that same system, and it was very powerful. The QSC can best be utilized as a monitor amp, or to drive additional speakers, but there is NO WAY to hook both amps up to the same two speakers, unless that BRIDGE mode is an option, and you NEED TO BE SURE about that. Very sure. You'll ruin the whole setup if you do it wrong.
As it stands, the fender should be stronger than the QSC, so you'll get the most sound output from the factory setup. The power rating on an amp can be greater than the speaker's, but you need to be careful never to push the amp too much. If you send 500 watts into a 300watt speaker ...it WILL break down eventually, maybe even immediately.
If you want more sound ... you need to MOVE MORE AIR. More wattage does not always translate to more sound. You want coverage .... dispersion....and clarity.
Fiddle with the internal amps alone for a bit, and if you still feel that something is missing .... maybe you can add a subwoofer and power it with the QSC. That will round out the sound and give you plenty of full, deep bass. It wall also take some of the load off your mains, and increase the efficiency of the whole system . More stuff to carry and set up though. You want that ?
The QSC was most likely intended as the power source for a second set of speakers, perhaps a monitor system.