I was mesmerized when I first saw Eric Johnson do this on Austin City limits back in 1990. It was so simple yet immensely effective. At the time I was (and still am) running a rack, being from the 80's and all, and was unable to rig a sound anything like this. He was using foot pedals and I was a foot pedal snob. Back then i thought, if I can't do it with a rack then I'm not gonna do it! Over the years I've added rack-mountable Loopers like the Line6 Echo pro, different Delays, units with pseudo sample/loop effects like the Freeze function in the Yamaha SPX 90 (and today the SPX 2000). I've bought and sold some foot pedal loopers like the Boss RC-20, RC-50 (and now the Line6 JM-4). The old green wonderful Line6 Echo Pro (gee I wish I had that one back,) The Boss DD-7 foot pedal does a nice job but they all have issues. They ain't a rack-mount unit, they don't have dual stereo in's, they don't save the loops for later recall, they don't take an SD card, they don't have a software GUI you can deep edit with via USB on your PC, they aren't MIDI-able.
They just don't make a rack-mount looper with all the stuff I need above. I just have to wait for someone to make one (well there's the Looperative LP1 unit but there's NO WAY I'm paying $1700 US for a friggin looper). I my hope is that Line6 will make one for me with all the crap I need, they come a bit closer every time they release a new one. here's a great site for info on real-time looping devices, http://www.loopers-delight.com.
Turns out when I was playing with my Pod X3 pro while on vacation, I flipped around a few settings and found you could toggle between dual tones and still have the delay of the old one sounding in the background. Once they were separated from each other, and the foot controller assigned to toggle a few items, the trick worked. He's a video showing the effect I'm trying to reproduce and how it was done on the Line6 X3 series.