I was going to say not possible but consider this:
-create a tag "compliant system"
-create a managed systems query for all managed system-1
-create a managed systems query for windows systems that match your criteria-2
-create a managed systems query for mac systems that match your criteria-3
-create a compliance query where the criteria is the tag presence-4
-create a server task to:
-run query 1 to clear tag off all system
-run query 2 to tag windows compliant system
-run query 3 to tag mac compliant systems
-run query 4 and mail/exports results
Its a long detour but it would work, makes sense ?
Basically like running two queries and doing a union, then running your query off of that...
Just fyi, that may not work as you expect.
My understanding is that task sub-actions are all evaluated at the task initialisation, sub-actions are not evaluated sequentially.
Sounds odd I know, but you may need to run these one after the other as separate server tasks rather than one with multiple sub-actions.
What i'm trying to say is that making sub-actions dependent on a tag cleared by another sub-action may behave oddly because all sub-actions are effectively evaluated together.
Just a heads-up