One reason he does this is to make use of the 10d B output on the SDD-3000s, as Daniel Lanois explained in the quote below.See the above picture - that will make the explanation a lot clearer.
Edge often uses the Korg SDD-3000 delay units; the TC-2290s, as used on ‘Streets’, are pictured above.
Sending the different delays to separate amps allows more control when mixing the songs -- you can vary the levels, or send 1 delay to the left and another to the right. There’s more info below about the stereo mixing for some songs.
The dimpled half is supposed to be where you hold the pick (it gives it a better grip).
Apparently Edge holds the pick either backwards or sideways so that the dimpled part of the pick grates the strings to sharpen the sound and give it a slightly grating punch - what some people call a ‘chime’ sound.
It’s simple - take a single guitar track that you’ve recorded from one microphone.
Make a copy of it and move the copy back by about 5ms (the simplest way is to just shift it to the right by that amount in Sonar, Cubase, or Pro Tools - whatever you’re using). Take the copy and pan it slightly right and pull it down a bit in the mix.
Most of us won’t ever play in a U2 tribute band or try to cover a U2 song in the studio, but discovering how The Edge uses his ‘trademark’ guitar delay is not limited to that.
How his delay sounds and how he uses them are a great resource for any serious guitar player.
He uses modulated delays which add a vibrato/chorus effect to the delay repeats.