That's the way I do it, on the few times I've used both. Assuming there's enough light to judge the effect of the CPL through the ND. Unless you figure out where you want it, take it off, put the 6-stop on, then reposition the CPL by memory.
Which is far too much hassle for me, and the reason I rarely use my CPL any more. That and the fact you get horrible banding on super-wides.