The easiest way to think about stereo width is this: Difference = Width. The more different your two sides, the wider it will sound. Different amps on each side, different guitars, different cabs, different EQ, etc. Eventually you'll hit a point where the mix sounds lopsided; for instance, if your left side has a mid-heavy Marshall tone and the right is a scooped Recto. At that point, you could remove some of those differences by, say, using two different amps but the same cabinet on both, and you can also correct the difference a little with EQ applied to one side or the other; scoop out the Marshall a little, or bump the mids on the Boogie.
As a starting point, I'd suggest finding two tones you like and double-tracking each of them:
Marshall L
Mesa L
Mesa R
Marshall R
Personally I'd pan the more-scooped of the two all the way to 100 and the other to 80, but panning is a matter of taste.
If that's not enough, try the same thing but with two guitars, one for each side:
Marshall L w/ Gibson
Mesa L w/ Gibson
Mesa R w/ Fender
Marshall R w/ Fender
If that's STILL not enough width, consider complementary EQ - a small notch on the left at whatever frequency you like, and a matching boost on the right. Then the same thing at a different frequency but with the boosts/cuts swapped. Nothing big, just a db or two to make the two sides a little more different.
For heaviness, there's always the trick Metallica used on Master and the Black Album - apparently they quad-tracked everything and then did another pair of takes just on the low strings, panned at like 40 or 50 to either side, just to make the chuggy parts heavier.
