Is it acceptable by US NEC standards to have two switches in a double wall box that operate on entirely different circuits in a late 50's house?
Yes, it is.
As long as other requirements for wiring at met, there is no problem with two circuits (or 3, or 4!) in a box.
Consider, in fact, that code requires this for dishwasher and disposal installs today: Split the bridge on a receptacle and wire each outlet to a different circuit.