I don't have a degree in electronics, and I am sure a LOT of the regulars know more than I, BUT: I'd bet a splitter would ruin your signal a LOT MORE than just using the Toslink to listen/coax for record.
Certainly there are commercial grade 75 ohm splitters available. We use quality splitters from Mini Circuits here at our satellite earth station but they have BNC connectors vs. RCA's that you would need. Video production parts vendors may have wideband splitters available with the RCA's? However there will be a 3db signal level reduction through the split. The same thing (-3dB level reduction due to the double-termination) would occur if you simply y-connect them via a simple jack & plug harness, which you could even make yourself if you don't want to buy a good one. The input impedances of your devices very likely don't change significantly whether they're powered on or off.
However I will leave it for others that are more experienced to advise you regarding any possible deleterious effects caused by the reduced data signal level. May or may not be a problem so why not simply try it & see what happens?
You can get them at Radio Shack, or if you want something more substantial, go to AudioAdvisor.com for gold ones made by Monster Cable ($20) --- I do not think they will ruin your signal, assuming the two cables coming off the splitter are not too long. I have done this on a temporary basis for a temporary setup and never had a problem.