Perpetual Technology P3 DAC & 2 Input Sources

I have a P-Tech P3 D/A converter with 1 coax digital input. I would like to connect an RCA y adapter to this input, so I can feed the digital output of the cable box and DVD player to the P3. I know only 1 source can be powered on at any given moment. That being said, would this arrangement be safe for DAC, and would there be any loss of sound quality or resolution? Thanks for your replies.
I doubt if there is any danger but the impedance effects are indeterminate. Try it or, as I would prefer, convert one of your sources to TOSlink or other. (Or add a P1A.)

The P3 will accept "coaxial S/PDIF, optical TosLink, AES/EBU, and I2S 5-pin mini-DIN connectors."