Use line out + line in combination, not the mic input.

Verify your output line out level. Crank it up to the maximum level can you use and the lowest input level in your input mixer with no distortion at peak. If you need to attenuate the input at the audio receiver, that's better. Don't amplify the input as it will amplify the noise also.

This might not reduce all the noise, but improves the signal/noise ratio.

PCs have poor ground design regarding ground audio noise. Even if both PCs share the same power cord, their share the same power ground at socket.

Audio cables also have a ground path, so you will end up having a ground loop between the audio cable ground and the PC's power supply ground.

Proper power ground earthing (thick wire, well placed ground electrode) will reduce de noise, but be careful about voltage differences between grounded chassis and a not grounded one.

If you leave one of the PCs grounded and the another not, to break the ground loop. Take special care not to connect the audio cable while both PCs are connected to power and running as it might burn your sound card.