I did it, and I think it looks great. Remember that GC isn't a wax, it's a wax/poly/resin blend. I'm sure that the NXT softens the GC and removes some of it, but whatever GC is left gives the NXT a darker, deeper look. It's a nuance, but if you're doing goofy combinations of toppers (like I have done) you're looking for nuances anyhow, so go ahead and try it and see if you agree with me.
Tom