Thank you all for the input.
As a couple of you have already pointed out, I do track each side independently. I do not copy the same take and pan. All that does is add up to the center, which I what I'm trying to avoid. Quad tracking, or having 4 takes of the same part is cool for sludgier, chordier sounding bits where you want density, but it doesn't really work on tighter rhythm parts.
What I notice in my mixes vs pro mixes on this subject, is that while I will have two takes panned hard left an hard right, they won't sound nearly as wide as in a pro mix. By the same token, the center instruments like snare, kick and bass, will seem more isolated in a pro mix than on my better mixes. Almost like there's a barrier in the center pushing the main rhythm guitars out wide. Do you think it's just a matter of better eq decisions?
In a way, but in my experience a number of other things are needed as well. What we are discussing is actually the MID-SIDE balance and certain stereo characteristics, such as spaciousness, openness, wideness, loudness, center clearity, frequency spread on each speaker etc. It's kind of a lot of things. As you know, the dBu/Voltage RMS curvature is cumulative, so most of the performance comes from processing the end of this spectrum efficiently, and this is something where pro mixes tend to stand out - you have multiple power conditioners, high dBu conversion, high dBu analog FET peak limiting/summing/monitoring and stuff like that. So yes, it is better handling of the frequencies, but to a great degree recorded and dialed in against a higher performance setup. Against this setup you then make all the mixing decisions and the what, how and why in regards to all of your mix balancing decisions will contribute to to what becomes possible. You do need hard panned guitars, but not the same guitar frequencies on both speakers, but rather different ones. That's the first thing.
The second thing is the volume * density of the lead instrument, such as the vocals. If you make the vocals too loud and dense, the guitars will sink into the vocal frequencies and the MID component will be more prominent in the MID-SIDE balance.
The third thing is electric guitar compression, which increases the density of these frequencies, letting other frequencies not get through in perception as easily, especially when you also add volume. So there are many things you can do. You should have a dedicated volume fader for MID and a dedicated volume fader for SIDE in your mix routing matrix, so that you can instantly do precisely this and nothing else, because everything impacts everything. Why you want to have those two faders separately is because you want to be able to adjust multiple effects with a single touch. For instance the SIDE fader should not only reduce the volume of the center panned sound sources but also reduce their density by lowering their compression wetness and maybe adding some effect like chorus on one of the sides. It is this kind of thing that actually dramatically clears things up in the mix.
I usually have the guitars hard panned and just increase the volume of them and at the same time lower the volume of the vocals and the bass guitar. When that clicks, that's done. Then I balance with the MID-SIDE faders, those have a bigger scope but can help further balance the two components. Then depending on what kind of characteristics that adds or removes, I might then compensate further which often yields an even better result.
So as you can imagine a whole range of things happen. In most of the cases it is the density and volume of the vocals that is the biggest cause of this. In my mix routing matrix I have a dedicated MID fader and a dedicated SIDE fader. These impact both volume and density. But I also have volume and density separated on individual volume faders on both the SIDE and MID. So a lot of my sound comes from the mix routing matrix. If I don't have these faders available, it's a totally different sound.
It helps a lot to figure out the impact of density * volume and this is a quite big topic in mixing. How you can best understand it is to grasp the concept of signal to background noise. When you have a sound source in the mix, the higher the density * volume of the other sound sources in the mix relative to it, the lower the signal to background noise is from the point of view of that sound source. So you can "improve" the sound of a sound source in the mix by increasing density * volume on it and/or decreasing density * volume of all other sound sources. How I have decided to deal with this in my mixing approach is partly like I said, I can control these things separately on MID and SIDE, which has a great impact, but I also work with these to create various characteristics. A high density * volume combo is not only dense, but what it really is in more creative terms is that it is more dominant. So I can create the dominance and "stand out" characteristics from this. Let's say I want the lead guitar solo to stand out and be dominant in the mix, then I increase the dominance fader of that and decrease the dominance fader of the other sound sources, but I do so using automation as well. This might seem complicated, but this is actually what you need to do and it is much easier to do it using just a few faders than not even having any faders available to do it.
So to sum it all up: I recommend that you create a dominance fader for all of the sound sources in the mix and balance these to make the side panned electric guitars stand out. Do this and ensure you do it against a high Voltage RMS solution, so that you can really hear when the mid range of the electric guitars cut through and so that you can really hear the details of all sound sources and so that the electric guitars don't cause any clipping.