Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Adding more loads on the outputs could do two things:
1. it could change the impedance of the total load on the output, and thus alter the impedance matching (of total load impedance to output impedance). The way to avoid this, is to have the amp for the 2 small extra subs have the highest output impedance possible, at least 400 Ohms, very roughly, would do.
2. it could possibly load down the total wattage of the Stagescape PA. This would, at severest, and at highest volume and thus peak output also, cause either clipping or else (perhaps) overheating or fuse blowing. So be sure you don't exceed the total Wattage of output, as a load by your speakers/amps/etc.