I've found that it's best to start with only one delay, by turning the Dly Mix knob fully counter-clockwise. Apply tweaks to the Dly A and Fdbk A knobs to get a feel for the basic controls.

Once you have settings you are happy with, turn up the Dly Mix knob a little and listen to how you can add the second delay in for flavor. You can keep the Delay B level low so that it adds a dimension without overpowering.

Turning Tempo ON (Press Encoder Button) is a great way to keep the two delays together rhythmically. With Tempo ON, try setting one delay to a quarter note and the other delay to a dotted 8th note. That's a classic setting made famous by U2's the Edge. There are many more note value combinations waiting to be discovered!