How to split 2 audio signals and then mix them?
Put 1 kΩ between signal 1 and audio in
Put 1 kΩ between signal 2 and audio in
Disconnect \$C_1\$ from audio in
Connect a wire from signal 1 to \$C_1\$


In theory you'll get 100% of signal 1 onto A0 of the Arduino. In reality you're probably going to get 99.9%, 0.01% being random noise and signal 2. I hope you can live with that.