[toni-a]

I do own two circular polarizers
I read to make variable ND I need a circular in the back plus linear polarizer in the front
however two CPL can be used if the front filter is reverse mounted, is that true ? would it work ?

[Brightcolours]

You do not need any "circular". Two PL's will do. You will get X pattern issues fast though, just like with other variable NDs.