Forgetting optical for the moment, with a coax connection you usually have a low impedance output and a high impedance input to the next device in the chain. If you put two inputs in parallel, there is a formula to determine what the resulting impedance (load) will be for the driving output. Many of the folks on this forum are spec-happy and should have no problem doing the math. Now see if the resulting load impedance is within spec for the source device and how the output frequency and phase response will be affected. You can probably easily see the difference on a scope just by running a tone or white/pink noise into the devices. You will need to take three readings - with load A, with load B and with both in parallel. The loads will usually have a Z of under 100K while the scope will be a meg or so and will not affect this reading, although it might introduce a bit of noise depending on your bench environment. This will tell you what is actually happening, but what Your and Your ears alone prefer is the deciding factor.
RCA Y- cable or is an A/B switch required?
I would like to connect my CD player to two different preamplifiers (one stereo connection for my surround sound receiver and one stereo connection for my vintage McIntosh gear). In the old days, we simply used an RCA Y-cable for each channel, but I am curious if the sound quality or a possible line level instability might arise.