[quote]Now the Band Im in plays in Drop C,and its only one guitarist,and I try to 'fill' the sonic space left as much as I can...[/quote]
The big question here should be "what is the guitarist is doing with his EQ?". If he's scooping his mids out, and over compensating with loads of bass & treble, which a lot of guitarists who detune do, he's seriously encroaching into the bass spectrum and killing the mix. Likewise, if you're scooping the mids too, you're both competing for the same soundwaves, and things tend to get a bit mushy.
If the guitarist cranks his mids and drops the bass/treble a bit, this should fill the gaps.
I played guitar in a 3 piece, and this all became obvious when we started recording rehearsal sessions, the bass & guitar were just competing for the bass spectrum, and the mids/tops were left a bit empty. On cranking re-evaluating the EQ, I cranked the mids, dropped the bass/treble, and bingo, next recording, both bass and guitar were clearly audible. Add some mids to the bass as well.
If nothing else, it's worth a try.