I don't understand how (why) this is supposed to work. If you don't record on one channel the memory is allocated to the remaining channel? Is that it? Why wouldn't the muted channel be looping dead air?
Do as Miko and I suggested: Set the DD-20 to "Pan" delay, be sure the unit is set to "Stereo output" (output mode 1 or 3 - output mode 1 is the factory preset/default; see manual p. 30) and plug both output jacks into some device with two independent inputs, like a mixer. (You can also plug in a set of stereo headphones and just take one earpiece off your ear to control the "channel mix.") Set the delay time to 23 seconds, feedback to maximum, effect level to maximum. Make an input noise. 23 seconds later the noise will repeat in the LEFT channel. 46 seconds later it will repeat in the RIGHT channel. 69 seconds later it will repeat in the LEFT channel. 92 seconds later it will repeat in the Right channel. Mute either one of the channels on your mixer and you will hear one delay every 46 seconds. You can do this with a mono input or a stereo input to the DD-20. In "Pan" mode, the DD-20 reduces the input to mono before creating the pan effect.
You'll understand it better (and probably giggle with joy at the free memory upgrade) if you just plug in and play as described. No laundry done, no groceries bought, no job, and probably a little drool on your Loopers Delight T-shirt. But Hey! 46 seconds of delay!