Can I send the signal from the Torpedo Live at the same time to my PA and to a speaker-cab?
Yes, the THRU, S/PDIF and LINE outputs can be used simultaneously.
When you plug a cabinet to the THRU output, the impedance of this cabinet is the one that your amplifier sees. In other words, with a 4 Ohms cabinet, you need to connect the 4 Ohms output of your amplifier. In this configuration, the load box inside the TORPEDO Live is disconnected. This way, you can override the 8 Ohms only limitation of the TORPEDO Live.