There is no passive resistance in making two wedding cakes.
Actually, there is. If an abusing couple insists on getting a cake or whatever from a fundamentalist Christian shop just because it's a fundamentalist Christian shop, and they know the owners wouldn't like to do it, and they're doing it precisely because they want to make the owners angry, then receiving two cakes at their wedding would send a very clear message to the couple as well as to the guests: "Here's your cake/whatever. We also feel you're being abusive, so here's another one to commemorate it. Have a nice wedding."
Even though I think it's pure assholery from service providers to be bigots, this would actually be an effective form of passive-aggressive resistance, done in all the right ways and, more importantly, in the spirit of the original.