Hi Andy -
This is curious, as I do not see any hint of a “double limb” with photospheric leakage with double stacking my two front mounted SM90's (image below)... rather it looks almost identical to your triple stacking image: The DS view you have above looks more like a typical 0.7 A FWHM image, and the TS looks more like a >0.5 A FWHM image.
You may want to experiment in getting your DS tilts and rotations optimized link
(for instance using a auxiliary refractor telescope), and then do the triple stack. The only other issue that might be involved is that if your DS mode uses an internal etalon, where the bandpass (and therefore contrast uniformity) can vary across the field of view/ image due to increased field angles, you may not be getting all the benefits of DSing in all parts of the image plane...