Having tried 2 methods, I'm not convinced you can get the dual radio input working, just use the white and accept that's the best signal you can get really.
I've tried the splicing two of the FAKRAs into one input, which lead to expected levels of noise and ghosting of sorts, slight overlap and delay.
Tried using a celsus diversity replacement kit, but it's explicitly for unpowered head units, the RCD330 Plus is still powered, so it was overloading the whole thing - again noise/static.
In the end the white antenna cable (roof) was the only one that worked.