ATTENTION !!! :

When connecting a second speaker to this output, the speaker is placed in parallel !!!

This means that the IMPEDANCE of the circuit is now HALF, resulting in 4 Ohms instead of 8 Ohms. This is not only on this system, it is in fact on EVERY in parallel combined systems .Make sure that your amplifier can take this  load, if not this WILL destroy your amplifier for sure !!