Why not book the two seats the same way a "customer-of-size" would? (which presumably must be done over the phone since I've never seen that option on the website) You wouldn't even have to lie. Just say something like, "I need the two adjacent seats to be comfortable." The phone agent will likely assume you are a customer-of-size and not want to discuss the issue any more than necessary. But they'd use whatever flag that airline uses to ensure two seats together.

In an oversold situation, the GA shouldn't notice/care that you have two seats as she will again likely assume you are a customer-of-size, and an FA who tried to reseat you after seeing you are skinny could be deterred with whatever documentation they give such pax--presumably a 2nd boarding pass.

I doubt TSA cares about these cases; they're too obsessed with shampoo, sandals, and mischievous pax who want to fly without ID. But if you get 2 BPs and one has SSSS on it, give the other one to TSA so you don't get the haraSSSSment of SSSSelectee SSSSecondary SSSScreening.