Hi, Don -
Will the XPro holder take the polarizer plus 2 sheet filters at the same time? I can't tell from the Cokin website...
Wanted to confirm this before responding...
I just tried the circular filter in the holder and was able to place one plate filter. The holder has 3 slots and uses 2 of those when using the circular. So the answer to your question is no - you can only use 1 plate in combination of the circular.
Hope this answers the question