Jack,
There is a chap on Astrobin who considered this same project. In the end he decided that an MX+ would just not cut it, so he bought an MEII see
https://www.astrobin.com/gqpdah/
Side-by-side is really the only sane solution! There are various ways to do this….via Bisque Versaplate accessories or Losmandy accessories.
The imminent MX Series 6 does have an increased payload, from 45kg to 56kg, but, even so, I think two SCTs of that size would warrant an ME.
Getting the OTAs in place and in balance sounds absolutely TERRIFYING to me!!
Martin