After I upgraded my PC subwoofer to optical audio input, I still had a problem: My sound card has only one Toslink output. But there is remedy! Toslink splitters! Hooray!
Those things have one input and two outputs, and entirely optical (passive). However they have a major flaw: They do not work! The plug conncetions and the prism have really bad optical characteristics and too much light is absorbed. At first, I thought the fibers were to bad, but I tried short (1m) high quality cables and experienced the same problem. Even cleaning the connectors didn't make it better, I figured out that one of the outputs was stronger than the other, it barely worked, the other one didn't work at all. A piece of junk.
You can buy active splitters, but the cheapest I found were around 50 euros, way too much money. I flirted with idea of building my own optical splitter, that would mean another box that needs power and an AC adapter though.
A lot simpler is upgrading the sound card to two optical outputs! I had still some Toslink emitters around, and I didn't really need the optical input of the sound card, so why not switch an emitter for another sender? The modification is quite simple: Just desolder the receiver, hot glue the emitter at the same place upside down (you cannot use the same pins!) and wire it parallel to the existing emitter. Just an aditional 100 nF capacitor and an 8.2 k resistor is needed (according to the datasheet i found).
Quite simple, but effective. Working like a charm.