Could you clarify? You want to use 4 side surrounds and 2 back surrounds? And plan to power the 4 side surrounds and your center and your fronts with the XPA-5?
If yes to all that then, yes, you will probably want to wire the side surrounds in parallel to 2 channels of the XPA-5. Not BECAUSE they will present a 4ohm load (although they will) but because this is probably your best option if you DO have to drive those 4 side surround speakers with only two channels, because the amp should be capable of driving that 4ohm load.
You COULD use two of the AVR's side surround amps for 2 of the side surrounds but, depending upon the relative power of the AVR's amps to the XPA-5's amps, you might have issues with the relative levels of the 4 side surrounds as you won't have a way to adjust the 2 pair of speakers' relative levels. But you are going to have issues with this relative to where you are seated, anyway. So, I guess you could use the disparity in output to your advantage. Unless your listening spot is equidistant from both pairs, calibrating 4 side surrounds on the 2 side surround channels is going to be problematic, anyway.
I do not think wiring all 4 side surrounds to your AVR's 2 side surround amps is a reasonable course given that the XPA-5 should be more robust. However, you didn't say what your AVR was. I am just assuming the XPA-5 would be more up to the task of driving the 4ohm load.
I assume the reason you are doing this is because you have a longish room or home theater, correct?