All I want to do is pressing O+P simultaneously which cann't be done by maestro alone. After mapping F12 to O+P with Karabiner elements, although maestro has been told to simulate the F12 keystroke, it does not activate O+P keystrokes.
When I press F12 keystroke with my hand, it does activate O+P.
Karabiner only captures keyboard presses from the specified device. You can see by going to Karabiner Elements => Devices to see which physical keyboard is being processed through karabiner.
If you run a keyboard maestro macro which simulates a f12 press, it sends... f12 key press to the OS. I can't think of a way that Karabiner can intercept a simulated key press. Maybe applescript key event can be your best bet to mimic a near simultaneous press.
Are you talking about true simultaneous press? or the keys o and p in rapid succession?
You can try fiddling with key down, key up with a delay of whatever ms you define in your applescript. I'm curious how you defined simultaneous key press in your karabiner. Can you show me your json snippet or edn?