Coaxial - USB from DAC

You cannot split SPDIF. And that is totally different than USB.

Not an answer to your question, but a dedicated streamer placed with the rest of your components will likely sound a lot better and solve the problem. You can feed it with a wifi extender and short Ethernet cable to avoid a new problem). My high end systems runs on a wifi extenders. 

You’re running 30 feet of USB? Is it an active signal cable? USB 2.0 has a limited usable length of just over 13 feet (4 meters) so I would think your data rates are low. Either way I would look for a better solution.

Can you not use an optical cable from your Zen stream to your dac and then use the coaxial input to replace the USB from the computer?

There are AES/SPDIF splitters available but they are typically for distribution rather than acting as an A/B merging two devices into one input. 

Got it, thanks-