Try EQing by ear. Have the other guitarist play a chord, then you play the chord, and try to EQ it to sound more like the other guitarist. Rinse and repeat.
If that doesn't work, a cut around 250 Hz and a boost around 2-3 kHz may help. But you really have to train yourself to hear it rather than using other people's suggested frequencies...there are just too many other factors for that to be really effective. Good luck!