Hi Mark, I am using a side by side set-up with 100mm refractor and 160mm astrograph and it works well. If the set-up goes too wide it will limit your viewing near the zenith as the scopes can hit on the mount. You may not have a real problem with what you are trying. I did try a 14" SCT with 100mm refractor on one side and the 160mm astrograph on the other; this did limit my views of the zenith. From my experience side by side is more stable than piggy back.
Side by side can, and dose work very well if you know your limitations, I had a great set up for imaging, and the only issue that might make it a little difficult is balance, but if one gets that right, it is the way to go.
