Once you figure out the song arrangement (and this is very very very very very important), you can than assign the instrument(s) that will fill this area.
When I'm mixing, and being given tracks that I did not record myself, I tend to pay hours of attention to the producers' intent in the arrangement, then it all seems to fall into place after that.
Panning sure helps this frequency area a lot, as well as using space FX (delays), dulling EQ on some instruments to make them seem farther, and lowering their levels as well.
When I'm encountering layered instruments (double guitars, et al) I tend to have one of them dominate a little more during parts, and the other during other parts of the song, thus making it more interesting to listen to, rather than a super-uniform blend of two instruments.
Cheers