There are a couple options with the gear you have:
1) Dual PA, already discussed, one possibly for vocals only, other PA for whatever remains. Avoids the comb filtering problem from the same source coming from two different cabinets close to each other.
2) Delays. Put the Yamaha's 'out in the crowd' and time delay them so they fire at the same time as the sound is arriving from the stage. You'll need the right hardware to do ~100ms delay for a, e.g., stack 100' away from the stage. And ideally a way to measure the delays for fine tuning.