I have explored many avenues with this, there are several ways to do it but only 1 is TRUE double tracked.
3) The only way to achieve a true double track live, is with another guitarist, backing tracks, or a looper. In this case ill explain how I utilize the built in looper on axe-fx. Hopefully you have the MFC foot controller. So, obviously you will have to play & record the part to loop it back first. I record/loop my riff then using one controller: IA switch, exp pedal, or scene controller. I assign the looper balance to go either full L or R and then do the opposite with my dry or main signal via volume block. Engage the controller and play along (same RIFF) with the loop (I personally like an exp pedal). Now your double tracking live. Note: Put looper at end of the chain and in parallel, mix 100, adjust level to liking. Also put the volume block at the end of the chain, in your main signal path. Might have to play with the L&R Pan parameters in Looper and volume as well. Sorry not in front of my axe-fx.
Lastly, I understand the need to have precision, and this way unfortunately doesn't allow for an immediate double track bc you have to record it first, but I think it's a cool effect live MONO to STEREO double tracked. Also, just FYI, you can do all of this with MIDI program changes if you have that capability. I've done it and it is PRECISE and effortless on your performance end. Otherwise you'll have a little bit of tap-dancing to do

if it were easy everyone would do it.
Let me know if you need more explanation, I can try and get you a patch screen shot or something.