The 3.5mm splitter or Y-cable works the best if impedance of both outputs are the same. I am not sure if you will hear anything on high impedance drivers if other one is low impedance.
So . . . you're confirming that the two PC sources will have their audio outputs merged simultaneously through the 5.1 speaker set? I also thought myself about the impedance factor, but both of the motherboards are "almost" the same model. I don't think that would actually vary across different motherboards! As I understand it, all of those 3.5mm jacks produce the same impedance. This is just a standard for PC sound connections.
Would I expect a reduction of volume from either input? It looks as though we're talking about three 3.5mm stereo wires -- green, black and yellow plugs or PC sound jacks. I'd only need to order some 6' patch cables . . . wait-a-minute . . . Now I'm looking at your picture of the splitter again.
I think this would require either two male 3.5mm stereo wires merged to a single female. For one computer, no patch cables would be needed, while for the other computer, male-to-female patch cables ~ 6' length would suffice for this.
I can only guess that such a splitter would also be available somewhere. I KNOW I probably have one or two similar to the picture you linked. But it seems to me I need the same sort of splitter, but with the two male plugs merged to a single female.
It is more of an advantage to allow sound output simultaneously from both computers than to deploy hardware that makes the output mutually exclusive between them. In other words, if one didn't want sound output from one computer, you'd just select the mute option at the sys-tray sound icon, or mute the virtual device defined by running software or speakers.
You're SURE that merging the two outputs would have the desired effect, without deleterious consequences? It's so simple, if such is the case -- I can kick myself for not having thought about it some time ago.