Thanks for the clarifications!
I’d tend to agree. IMO, either %for or %every (as @tw-FRed suggested) communicates more clearly that each output of the outer filter is tested against each of the inner filter conditions.
It’d still be helpful to see some more “real-world” applications if you’ve got any in mind, though. We often hear that there’s already a lot to take in, syntax-wise, so I do worry a bit about introducing new patterns without sufficient motivation — especially if they’re visually similar to extant constructions like %if, which is explicitly not iterative. I think we’d want to demonstrate a significant advantage over simply nesting <%if ... %>...<%endif%> conditions in a $list — or, if there is no real advantage but brevity, demonstrate that this pattern would be used often enough to warrant the new syntactic sugar.