Just a minor correction: I'm able to use the same Privacy card for x merchant for any number of y merchants, so long as it stays under the charge limit I've set for that card.
I've only experienced this with Kickstarter and then a secondary payment collector (BackerKit, etc) where some extra charge had to be finalized, typically shipping or maybe I threw in some extra doodad, and then had it fail because that second charge wasn't technically the same merchant.
But this was fine and I was happy - working exactly as intended. I just created a new privacy card and updated payment.