You record the inputs of each player individually, and in the example, you can only record the inputs of the first player. I guess I didn't make that clear enough.
The crash seems to be something I overlooked, sorry about that!
I'm noticing now that the white ball in my browser quickly walks itself off the screen when I load this page and my normal controller controls player 2 in the browser even though it properly controls player 1 in GMS2.
I did test this in GMS2 before but wasd is mapped to the second player too, along with the mouse click jump so I assumed that was the first player and the white ball was something else. I also see now that the color of the text corresponds to the ball and only the white text has the record info. Got it! Thanks for the info.
Also, may I suggest publishing a package with just scr_winput and an example obj_input_controller that can be imported into existing projects? I just created one for myself but it would probably be useful for others and for updates.