If the razor is empty when I select it from the rotation, I put in a fresh one. If there is already a blade in the razor when I pick it up to shave, I know to toss it when I'm done.
+1 to this. I, too, use them only twice (in general) and do exactly as you describe. If I am using an expensive blade, I will generally reach for a third shave, but to identify what will be the third trip, I loosen the head or put the razor on the rack with the doors open, to remind me that the blade should depart the razor when I'm done.
-- Chet