Sure, but as soon as you create linkage between them - if the state of machine A affects what transitions are possible when in machine B and vice versa - then y

Sure, but as soon as you create linkage between them - if the state of machine A affects what transitions are possible when in machine B and vice versa - then you don’t really have two state machines, you have a single Cartesian state machine combining both. Or you have a pair of metastatemachines which aren’t able to be analyzed in the same way as an independent state machine. It’s like the difference between solving a maze, and solving a maze with constantly moving walls and traps in it.