Layering Leads

I am curious about the techniques that recording rock and metal artists have used in their studio recordings to give their leads a thicker, fuller sound, but keeping the exact same melody. An example I know is that in a few of Metallica's old tracks, they played the same lead on two guitars at different intervals (usually a fourth apart). I have heard other techniques used that I can't figure out how it's done. An example of something I cant figure out would be The Scorpion's "No one like you" They have a multi guitar lead. Can anyone else tell me about a successful used formula that Im missing..perhaps with seperate channel delays and such. I know Randy Rhodes recorded his leads 3 separate times for the left, right, and center channels on some of his tracks. If anyone else can share their wisdom with me, thank you!

Well, they have gizmos called harmonizers, that automatically play a fourth or a fifth above whatever note you play. And there's a technique you can do with delay, so that each note you play is overlapped by the echo of the preceding one.

Is that what you mean?

Thats not exactly what I meant, Im aware of the effects methods...a harmonizer such as the Eventide 3000 is very nice...expensive, but nice. I was more interested in the recording techniques using multiple lead guitar layers. Agreed you can use effects, however the effects tend be 'robotic' in nature in that they basicly shadow EXACTLY whats put into them, whereas a manually replayed line that is say delayed will have subtle differences that add characteristic style to the line. I was wondering is anyone had interesting techniques that they would like to share.
I'll give another example
On the GnR track "Nighttrain" Izzy is playing rythem out of the Left channel...Slash is doing Rythem and Lead out of the right channel...but they mesh so well together that if you were to turn one of the channels off..it would be a totally different song (try it sometime)....anyone have techniques?

The key here is EQ.
I'm not a nerd, so I don't know the terms, but usually, you'll end up mixing a tele with a les paul on the same track, since they don't have the same frequencies, and tend to blend together... If you don't own 545 guitars, like everyone else does on this board (I hope so) you might want to tickle your pickup selector or your tone control before re-recording...

I don't think there is a formula written in stone about acheiving this type of composing.Start out with your main riff as the foundation.Decide what key it's in and all the relative notes and chords that belong in this key.Experimnt with different combinations until you get the ideal blend you are looking for(sounds like mixing your favorite drink).AC/DC's highway to hell and Slayers Mandatory Suicide are a couple more examples.If you really want to get nuts, go outside of the key but sparingly(Primus comes to mind) or it can get nasty.

I think their are several approaches that are commonly used, Unison, multi tracking the same part 2 or 3 times, like L said, different guitars, amps, pick-ups settings could help alot, also mike placements, Jimmy Page used the old distance makes depth formula. Octave, same part, an octave higher or lower. Intervals, most common 3rds, 6ths, occasionaly 5ths or 4ths, something Neal Schon did on Journey"s Infinity album ( I think it was Infinity ) on the song Ides of March?, memory is a tricky thing sometimes, was to record the root, then the 3rd, then the 5th, so you are playing a triad, which will sound quite thick, Something you might want to check out is Benoit's webpage and his piece called Renaissance, if that's what you are looking for, I'm sure he would discuss it with you...

Thanks for all the replies guys. I'll have to experiment with some of this to see some of the different tones I can create.