WOW FML
Six Flags America called me, and said the same thing. Because my season pass is already activated at Six Flags America, if I buy a premium dining pass, I won't be able to activate it or use it Six Flags Magic Mountain. The problem is, SFA doesn't open until April 6th, and I'm going to SFMM in February.
My plan is to buy a Premium Dining Pass and try to get it activated at SFMM. If they refuse to let me have it, I'll just activate when I get home at SFA anyway. UGH