Let me add something important for true ADT sound. REAL ADT means the track was already laid down on tape BEFORE any ADT process.
The Engineers were playing it back through multi-head tape machines, and the second copy (modulated) was laid out from the original, to emulate a second performance.
As I understand it, that also meant a second track (because of multiple tape heads and sources you could tap) could in theory also be varied to play slightly AHEAD of the original track laid down, not just behind it. If you ever get the WAVES plug-in you can see what I mean - visually, the tape heads move both left and right over the original source track, by milliseconds.
For us live players, that is impossible to do! Because we are playing our track live, the ADT second track effect can ONLY be a delayed sound, behind our original playing.
Once we lay down a recorded track into a DAW though, in processing it is theoretically possible that the second track can be sometimes be shifted ahead and sometimes behind our original track with modulation, which is why plug-ins like REEL ADT can do this.
Now -- you truly CAN get some amazing wide doubling effects with experimentation and great guitar thickening in the AXE FX, it's killer. Ian has it right!
But to get the real "Abbey Road" Beatles type of ADT sound, that is very hard to do because of this issue (the AXE FX is amazing BUT it can't anticipate what you will play before you play it to generate that second copy modulating both before and after the notes you play!).
As I understand it, engineers look at 24ms and 28ms as magic numbers in terms of setting delay for double track. (Try it, pan dry one side, then 100% wet delay with 24 or 28ms to the other, see what you think -- hear that Beatles-esque guitar bathroom reverb sound?)
But when I look at WAVES Reel ADT plug-in, it is varying by up to 15 ms BEFORE original source track (left of original tape source head, so tapped from a tea placed before the main playback head) all the way to 15 MS AFTER track (right of tape head) -- which is basically a 28-30 ms total span...
Overall, that's why I love the WAVES REEL ADT plugin - it really emulates the Abbey Road process. (BTW I have no affiliation with WAVES).
Perhaps I''m not technically right, that's not my forte, but I wanted to lay out why this is such a challenge to nail it dead-on in AXE FX.