Joey raised one of the main points - playing different things.
There's a concept in recording and sound reinforcement called masking
. Masking is when two sounds blend together and become one sound to the listener. Sometimes you want masking, such as with the bass and the kick drums, and sometimes you don't - like with two guitarists.
To increase separation (the opposite of masking), you have to look at a few things:
- Timing - when things happen at different times.
- Pitch - playing things at a different pitch, such as different chord voicings or playing harmony to a melody part.
- Panning - having different sounds come from a different direction. Mostly applies to stereo mixes, which are not usually used live. Has some application to smaller stages, where the guitar amps form a large part of the sound reaching the audience. So putting amps on opposite sides of the stage helps with this, but you run into the same problem as with stereo mixes - people on one side of the room hear a different mix to those on the other side, and only folks in the middle "sweet spot" get to hear the mix as it was intended.
- Tonal spectrum - (very important one) many guitarists playing together have a tendency to go for the same type of sound as each other (in rock, typically scooping the mids and boosting high and low frequencies). This similar tone makes the guitars mush together. If you take one of the guitars and do the opposite (in my example, boosting the mids and cutting highs and lows) - the two will immediately stand out from each other, but fit together extremely well. It's often a "natural" EQing - where the guitarists will use different amps (like a midrangey Vox and a Fender with naturally "scooped" tones) and/or guitars to get different sounds that lock together without interfering with each other.
- Tonal spectrum part 2 (thou shalt not shit on the bassist's sonic space) A lot of guitarists also make the big mistake of boosting the lows and extreme lows to make the sound heavier. Problem is the tone starts interfering with the bass guitar. Bass frequencies are where the mush starts happening - you'll notice the first thing any competant sound engineer does with a mix is switch in low cut filters (usually between 80 - 125 Hz) on most of the channel strips: guitar, vocal, most toms and the overheads - leaving just the kick, low toms and bass with very low frequencies. This immediately cleans up the entire mix no end.
Also, just generally - if you have more than one distorted guitar, you should find that it's easier to get the heavy feel with less distortion. Take a listen to Led Zep recorded vs. live: Page (a masterful producer/engineer) would use more distortion live to get a heavy feel, while in the studio recordings - which were lots of overdubbed guitars - he would use less drive and distortion on each.
Stage volume (special mention) - if you have a stage setup where you can keep the volumes lower: smaller amps mic'ed up, plexiglass iso screen or baffles for the drummer, etc., the sound is controlled more by the sound system and there's less leakage between mics. This also helps the sound quality immensely - not to mention you get to be able to hear when you're my age. Lets face it guys, If you're not playing on a huge stage in a massive venue that can absorb the sound levels, having two full stacks wound up full ball on stage is just stupid, and will do nothing but screw up the sound and irritate your engineer - and first rule of live sound is do not irritate your engineer
- he has power over you a lot of the time. Work with him and he will make you sound good, work against him and he'll make you sound like shit.