Can you have grand mastery with multi class?

Perhaps. You can get grand mastery in dual class. But why don`t you download shadowkeeper and try? :)

I don't see he asked for cheats cirrostratus.

No, you can't get more than ++ without pure fighter if multiclassed.
Only what you can is to betray your race, play as lame human fighter and after you master a weapon, you can dual him into something. Ain't that racism?

Racism? Nope. Brilliant? Maybe. Useful? Definitely.

:)

Fighter level 9 dual classed to Cleric is rather usefull as long as you remember to specialize in one of the cleric weapons. War hammers or flails will do great