Well for one thing, when you solo, try not to go too high pitched, cause that really does make a big vacuum in the middle of the sound. I mean if a song calls for it do it. Also you can have the bass player add a little distortion to his tone, that way filling up some of the space of the guitar. Also have your bass player play more chorded riffs, that also helps fill up the sound. And maybe lower your volume or have him, raise his volume up a little, that helps some too. Also when you play live, the mixing of the PA system can add a lot of stuff like reverb and compression which helps fill in the gaps when you start to solo, the only thing is the person mixing needs to know your songs some so he knows when to add/take off the effects.
I mean a check out Decapitated, they have a really nice full sound and only 1 guitar player. Another good example is Pantera, who manage to stay heavy and still Dime throws in some crazy solos.
Hope this was a help, keep thrashing.