Problems are almost always better fixed in the mix than in mastering. When two instruments are competing for space in the frequency domain the solution is usually to EQ them out of contention. This is the main reason why the "make every instrument sound it's best soloed, then mix them together" approach doesn't work very well. What works best for a track by itself often sucks in the mix, and what sounds great in the mix often sounds lacking by itself. A parametric sweep on the main buss with a narrow band boost will find the offending frequency, then you can either remove some of the offending freq from one or the other, or both tracks with a static eq adjustment, or you can automate the eq fix for just the collisions, or you can employ a multiband comp (just use one band) to dynamically fix the offending frequency. The multiband could go on a sub buss with the two tracks routed through it, or on one of the problem tracks, or one on each of the tracks, or even on the main buss. Experiment and see what works best, including Johns suggestion to simply edit the volume curves to duck the problem.
Regards,
RD