If I'm understanding this correctly the only way to do that, that I'm aware of, would be to set the footswitch to toggle up or down (but not both, maybe with a long press you could get it to do preset up and a regular press to do preset down, but this is going to be controller specific and maybe only possible via midi).
If you are toggling between presets theres going to be a lag in audio as the Stomp is loading all of those blocks, etc.
I just found the solution!
While I'm in "preset 1" load up command center and tell FS4 to load "preset 2", save the preset.
Then, when in "preset 2" load up command center and tell FS4 to load "preset 1", save the preset.
Now, whenever I hit FS4 it toggles between my two presets!