Think about how a stereo speaker system reproduces a mono sound.
A sound appears to come from the phantom centre when the left and right speakers reproduce the same sound at the same level.
The brain decodes this and hears the sound coming from the centre.
If the left and right channels are very similar you will decode this as coming mainly from the centre.
Impulses or amp sims don't help this because they usually use a static snapshot of a speaker simulation.
The bigger the difference between the left and right channels, the wider the signal will appear.
For double tracked guitars in metal, centre shifting can be a problem if you vary the chain too much for left and right guitars.
The best ways around this (for me) are
1. Use 2 different heads and tweak the second one to sound as close as possible to the first one.
This usually gives a strong stereo image and prevents centre shift.
2. Put mics on 2 different cones and use one for left and 1 for right.
3. Use 2 different mic pres for left and right (eg ISA left, API right)
4. Use 2 different guitars. This can work but usually intonation is better if the same guitar is used for left, right.
5. Use a sum of 2 mics (eg 57 and 421 or M201) and use more of the 57 on the left and more of the other mic on the right.
I tend to use the same cab for left/right. I find changing cab gives a far more dramatic difference than any of the above.
If a band has 2 equally tight players this can give a good stereo spread too. Often though you will get one much better than the other.
Here is a mix from today.
https://copy.com/YpG7rwfNTDg6cvmf
ESP Custom Eclipse to mesa MK left and Marshall DSL right.
Once the left MK track was down, I tweaked the DSL to make it sound as close as possible to the MK4.
The DSL needed a tubescreamer to get the right attack.
They sound different enough to be wide but not enough to cause centre shifting IMO.
Both amps going to recto cab with 57/421.
