Please check out the Phase Widener, which is mono compatible.
The point of a Double Tracker is to delay an opposing channel in a two channel (stereo) signal and add some modulated offset to sound like two separate guitars playing. You can't do that with a mono signal unless you record in stereo first and merge the two channels together in mono afterwards. Trying to DT in mono comes out like mush.
When you do real double tracking e.g.
Record one part with Guitar A
Record the same part again with Guitar B
Maybe some EQing. Delay, Panning, etc, is needed for these tracks.
When you now playback the two parts in mono this should also work OK. No as good as in stereo I guess.
I think this is also the setup for live. Maybe the two guitars are a little bit left right/panned.
I am not experienced with live setup of doubled guitars, so please excuse me when I am wrong and help me to get it right.
If you have two guitarist live with their own amps then they are naturally in stereo. If they go to FOH, then the sound engineer should pan them accordingly to the depth size and width of the venue. A wide venue is not going to appreciate stereo that much. Audience members on one side of the venue will not be able to hear one of the guitarist well with hard panned stereo - and that's about 25% of your audience.