It doesn't seem like you know what you want from mastering, or understand what it is.
What mastering actually is:
The final stage of setting the tone and flow of a body of songs. Occasionally you hear that someone 'mastered' a single track. I've even been credited with that. It's not possible. It's about the flow from one track to the next and creating a cohesive whole from the different songs.
What people think it is, and think it is so often that this mistake has become the new truth:
Making mixes sound 'better', and making them louder.
About your 'pre-mastering' idea; something not too far from this does actually happen. If, for whatever reason, a mastering engineer requires more control over the sound than is afforded by a single stereo mixdown, they may request 'stems'. These are summations of the guitars, bass, vocals and drums (or elements of the drums: kick, snare, toms and overheads might all be handled separately). They will be treated individually such that a better master can be achieved. This only happens when the mixing, engineering or both sucked and the mastering engineer has to pull out their mixing skills (all mastering engineers can also mix, usually better than mixing engineers) and fix the fuck ups. Whatever they do to those stems is, however, still just mixing.
Track and mix very quiet. Leave yourself lots of headroom on your master buss. You can force yourself into a position where you're carrying out actions that fall into the skillset of 'mastering' by simply making shit too loud. You should never apply anything to the master buss until you actually want to.
The vast majority of sonic issues and effects can be achieved in the mix. There is a certain gelling effect to applying compressors and EQs to a master buss that can't be achieved in the mix (and applying reverbs to masters, if that's your thing), and that's part of the mastering process (or what it's become at least), but they are (or should be) comparatively subtle. Alter the sound of your mix in the separate tracks or instrument busses rather than in the master to whatever extent you can. By the time your mix is done, your sound is 95% finished, if you played engineered and mixed well, which leads to...
Put as much effort in up front as possible. Recording, when done well, is a front-weighted process. Better playing makes engineering easier. Better source sounds/engineering makes mixing easier. Better mixing makes mastering easier. Playing > Engineering > Mixing > Mastering. If you're deferring something to a later stage and you don't have a very good reason for not doing it earlier, you're doing it wrong. For example 'I'll clean up that muddy guitar in the mix'. No. You'll clean it up in your technique first, engineering second, then mix. 'I'll brighten it up in the master'. No, you'll brighten it up in the engineering (or mixing, if you got stuck with dull sounds). _insert any number of other examples here_
Experiment with different approaches. What my venerable colleague (counterpart/compadre/whatever) Matt describes is one perfectly valid way to go about it. I, on the other hand, at some point decide ITS TIME and stick a comp and limiter on the master buss then mix into them from then on. I therefore mix and master somewhat in parallel, but the overwhelming majority of what I do to affect the sound (once printed) is in the mix, not the 2buss. That we do things this way doesn't mean you should. There are a number of other ways.
Bussing mix elements up and treating them as a single whole is fine, but it's nothing to do with mastering. I often dual and multi-mic guitars, and usually quad-track. I'll create a 'guitar mix' which is the balance of the mics that I want to work with for each guitar, then the balance of the guitars that I want to work with for the mix, and then I'll mix with that sum. I'll do the same thing with multiple bass tracks - taking a DI and splitting into (for example), bassy-bass, clangy-bass and distorted bass, setting the balance I want between them, and then the total is The Bass. It's just called 'bussing', not pre-mastering or anything like that, and it's entirely normal. If you send them to an engineer later then that kind of thing will be magically re-named 'stems'. I don't know why. It's odd.