Stereo line level to mono microphone level
Yeah, you should be able to.  The linked answer has a single channel line to mic level attenuator, with a 10K series and a 100 ohm parallel resistor.  If you want to make two inputs, change out the 10K for a 20K and put two of them in parallel, one for each input.  So for the circuit you posted, just replace the 470 ohm resistors with 20K and the 20K with 100 ohms.  It won't have the same impedance as a microphone, though - the impedance will be a bit less than 100 ohms.  I believe a microphone is 600.  So if that might be an issue, just multiply everything by 6 - use 120K and 600 ohm resistors.