I had in mind the CS90/12 as it does well bifurcated.
Because the fsa of 2x64mm ~/= 1x90mm fsa the scale shouldn't need to change by any large amount.
But the conversation in Jumpy's thread being Bifurcated would be hinded on a 25/39 because the nacelles being set further apart.
Since you have model in hand in your estimate is it doable moving it up and slightly back without creating agreesive angles to the bifurcated duct.
With your current 64mm scale what did you make the inlet area or fsa% and the exhaust fsa%.
I think I missed why the scale would have to be larger to handle the 90mm.
In any case that twin 64mm is the perfect size so I would be more inclined to fudge the nozzles than make it bigger.
Small scale question; are you going to add a small OLS in the nose? Small sheet of clear plastic with a keychain cam behind it? It'd be the best view in the house