I have two computers with a pair of speakers each. The speakers are
standard MM speakers, powered with a sub that serves as amp. I want to
use the speakers on one computer as the rear set of surround sound
speakers on the other computer, and still have the computers be able to
send to their respective speakers.
What I have designed is a switch box that allows me to select which
computer signal goes to which speaker set, with diode protection on the
inputs from the computers to prevent backfeed.

My question is twofold - First, are the speakers going to be able to
handle the dual signal from both computers? I can't decide if the amp
will handle it or not.
Second, do the grounds or cable sheilds need to be diode protected as
well, and if so, which direction? Toward or reverse from the input
signal direction?

Thanks.