Correct, I am doing exactly that with my 3,1 and costabunny did certainly also do it with her 5,1. I think she has sold it since but she posted the solution on this thread:
https://forums.macrumors.com/threads/1675846/
With pictures.
And I did likewise. You do have to setup bootcamp in the sled and make sure the Windows loads the standard AHCI drivers for the x2 when it is empty. I had both SSD's in the sleds, powered down, moved them both to the X2 sticking it on top and it boots black screen legacy mode Windows 7x64 in SATA3 off the second port with OSX on the primary on the PCB holder.
The only 'hacks' were the thermal sticky pads and power extension though Apricorn obviously think it can't be done. Of course not