How are you splitting PMax and standard shopping without cannibalising?

Client runs both and I can't get a clean read on which one is doing the work. The theory is that PMax wins the auction when it expects to do better, so the overlap is fine and you should judge the total. The practice is that standard shopping volume fell off a cliff the week PMax launched and combined revenue is flat, which is a much less flattering way of describing the same event. Three approaches I know of: 1) Run both and accept that PMax takes priority. 2) Split by product margin - high margin into PMax, the rest in standard. 3) Turn standard off and stop arguing with yourself. Has anyone tested more than one of these on the same account? I am wary of drawing conclusions from a single before-and-after when seasonality is doing half the work.