Why are you fixing a waterproof membrane with waterproof adhesive?
Schluter over-kill at work again!?
I would fix using a standard adhesive - as they do in the States.
Unless you're totally in need of a guarantee, then you'd have to use kerdi-coll (slow-set).
Personally, I'd always lap my corners and joints with kerdi-fix these days. Far easier to do and still effective - although I know Schluter would say "not the correct way".
Ardex WPC would work, but why bother? Either use the full monty or use standard adhesive imo.