Over under or side by side?


Hi group,
I have been imaging with a 600 mm fl refractor mounted piggyback on a 800 mm fl reflector with my AP 900.
I can do 12 min subs with the refractor before flexure and or mirror movement in the guiding reflector starts to show up in my images. I have another refractor that I would like to replace the reflector with as the guide scope to eliminate any mirror movement. An OAG is out of the question on this particular imaging refractor because of the corrector and back focus required.
My first thought would be to purchase the AP side by side dovetail plate to mount the 2 refractors. My question is ; is there any disadvantage to a side by side configuration vs. an over-under for autoguiding?

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