It would work but you need much smaller resistors then I originally stated. I was thinking the signals would be going into an amp, but they're going to low impedance headphones.
I assume that you have tried this and each works ok separately with headphones? That is, the outputs are capable of driving a headphone and are not just low level line signals.
If so, try about 10Ω resistors in series with each line out from the XBox and one resistor each in series with the two lines breaking out from the Controller. The XBox and Controller have low impedance outputs, so the resistors will isolate the stereo signals from being mixed.
You will lose some volume that way, but that's the only way unless you want to add an external amp for the headphones, and use my original resistor values into the amp input.