Sorry to necro this if it's too late, but I was in the exact same boat, and bought the Lian-Li Lian Li O11D EVO XL - it's one of the few cases that can truly thermally isolate two GPUS, particularly if they're pass-throughs, like FEs.
OCUK sell them for a bit under £200, which is pretty decent value. The mounting kit to separate the GPU is £6, but you'll need a good Gen5 riser card (suggest a 50cm one with a vertically side mounted PCIe slot facing the rear of the case).