(even easier, you could just connect straight to the second unit and take analogue from that to your current amp – I doubt you would be able to tell the difference between one D-A converter and another, particularly if you’re playing mp3s or any other compressed audio file).
I’ve had a device with a combined 3.5mm Jack/optical output.
I don’t think you’ll get a splitter for the optical output as the light loss would be well over 50%. (Split two ways plus leakage)
Edit: I stand corrected, although I suspect the link above is a repeater as it’s got a power supply. However it won’t have a 3.5mm analogue output for headphones.