1/8 inch to regular speakers?

What kind of speakers and why do you not have a receiver anymore.

You need an amplifier of some sort to drive the speakers. I would recommend buying an old used stereo receiver on eBay. Should easily be able to find one for under $50. It will have stereo RCA inputs. You would get a 1 1/8"-to-RCA adapter, connect the Mac to the receiver, and then the speakers would connect with speaker wire.

Your speakers need amplification. The Mac output does not have amplification (beyond the miniscule wattage that headphones need)

So whatever you come up with has to solve that primary requirement.

Asking the same question different ways does not alter that fact.

Yes of course you can have them both playing.... IF they are driven from an amplifier or receiver of some sort. A speaker switch doesn't change that.

You need to sort out how to get the output of the Mac to the receiver, and then wire both pairs of speakers to the receiver.

Or, get an Airport Express if you have Wireless in your Mac, plug it into the receiver, and send the signal to the receiver wirelessly.

Or, get another receiver or amp for the computer room (wait, bathroom? whazzup with that - you use your Mac in the loo? )

Or get powered speakers for the computer and forget the Yamahas.