Chris,
FYI, I am using Canon 60D and 100-400mm V2 on an ADM side-by side vixen.  Astromania 60mm on right side,  Camera on left.  Very solid. Side-by-side adds 2 lbs to mount.
I could not find a shoe adapter that didn't allow anything mounted on it to not wiggle side to side.  That is why I went to a side by side adapter.  I use Sharpcap and it works very well
/Jim




Mounts:      ES EXOS2-GT w/ PMC-8
Lens:          Canon EF 100-400mm f/4.5-5.6L IS II
Cameras:   Canon 60D Astro Mod, Canon 7D2
Misc:          USB-to-Cat6 Extender, Astromania Red Dot sight (Flash Shoe)
                    TELRAD, ZWO ASI120MM Guide Camera, Astromania 60mm Guidescope
Software:   EXPLORESTARS IOS & WIN, Stellarium. Backyard EOS, StarTools, APT, DSS
Computer:  2012 MacBook Pro Bootcamp WIN 10