If I connect an external speaker with an 8 ohm impedance to the speaker out connection, does it cut out the two built-in speakers? In other words can both the built-in speakers and external cab be active simultaneously?
Both speakers - internal and external - will be active simultaneously.
So the external cab is always additional. (and must be 8 Ohm for a resulting min load of 4 Ohm).
You can also disconnect the internal cab by removing the SpeakON end, and connect an external 4 Ohm cab directly to the head (not to the connection on the cab itself), or 2 x external 8 ohm cabs.