Yes, all objects are touched/clicked regardless of the Z-order, even if they are invisible. You can put all UI elements into a family and create your event like this:
+ Touch: On tap gesture
+ Touch: [X] Is touching Fam_UI
-> Sprite: Simulate Platform pressing Jump
But i dont know what you mean by changing characters.
Thank you, kriand! I will test it later.
Sorry, I forgot to mention that I want to alternate between two characters of the game with a double tap on the screen. That's what I meant by "changing characters".