That's a very interesting question, and one that I asked (in relation to optical inputs) a long time ago here on this very BBS if I recall correctly.

Here's the situation:

The devices in question here are using optical (fiber optic) cables for their digital audio output. This is, as you might expect, a bit of a different animal than an ordinary bit of copper wire might be. The fiber optic cable is a continuous digital data channel that modulates a diode light to deliver high bit rate data. It's more like a network cable than an audio wire. In that situation, you might not expect that a Y-adapter would work any more than you would expect to be able to have a telephone conversation and a fax transmission on the same phone line simultaneously.

But even then, the truth is that, yes, you CAN have a Y-adapter for optical audio cables. In fact, I'm doing that very thing: Two of the optical output devices I'm using are sharing an optical input on my receiver via just such a Y-adapter. They are game consoles, so I wouldn't ever have them both turned on at the same time, so there's no issue with the signals overlapping. That's what makes it work, by the way: In order for the Y-adapter to be possible, only one of the devices can be turned on at a time.

The problem is: Some devices don't share well like that. For example: one of the devices that I'd like to have sharing a Y adapter is my Tivo DVR. This thing leaves its optical input "live" (lit up, red diode light coming out of the little hole) at all times, even if the DVR is placed into standby mode. Because it does this dastardly thing, it cannot share an optical input into my receiver; if I want to use its optical output I can't use a Y-adapter on that device. This is one of the limitations that I'm up against with my TV system's wiring, and one of the things I was hoping the HDMI inputs on the TV would solve for me. Ah well.