The Y splitter is parallel so it will have same voltage on each leg, Same voltage made by the Dac9.
However, the current produced by the dac9 must be shared between both of the loads being driven.
This is where impedance comes in. Use an online parallel load calculator to make sure the source impedance of the dac is at least 5, preferably 10 or more times lower than the net load impedance. Most modern solid state output stages have <200 ohm output impedance, and most audio input jacks are >5000ohms, usually >10,000 and sometimes 47k or higher, so you should be ok. Just run the numbers to know for sure, or listen for loss of bass. Lookup the output impedance in dac9 published specs.
