share eletromechanical buzzer between 2 mcu's?
Does the high side-driven version of the buzzer still use a PNP transistor? If so, the MCUs could use open-drain outputs in a wired-OR configuration. Otherwise, you might need some glue logic. Just switching VCC and/or ground won't do it; that'll just disable the buzzer.

Your circuit on the right won't work. You can't drive the NMOS and PMOS gates with the same signal since one of them will turn off. (Also, the source of your PMOS should be on the 3V side.) But you shouldn't need to switch both power and ground. A low-side NMOS to switch the ground would do what you want, but again, I don't think that'll let you control the buzzer.