Various coaxial SPDIF splitters employing active (powered) circuitry are available, but I have no idea as to the quality of the signal waveforms they would provide to the two destinations, and consequently the effects they may have on timing jitter at the point of D/A conversion and/or on digital noise that may be introduced into those components.
I would not use an unpowered device such as an RCA y-adapter for this purpose. It won’t hurt anything, and it **might** function, and it **might** provide somewhat reasonable results depending on the design of the specific equipment. But it would result in a drastic impedance mismatch, which at the RF frequencies that are present in SPDIF signals would result in signal reflections that stand a very good chance of degrading sonics. (See the Wikipedia writeup on
"characteristic impedance"). And while I don’t doubt that some audiophiles have done that, and perhaps have achieved reasonable results in some cases, at best it would be very poor practice from a design standpoint.
+1 to Erik’s suggestion.
Good luck. Regards,
-- Al