The headphone outputs on the 18i20 share their source audio with whatever is assigned to Line Output 7 and 8 (for Headphone Output 1) and Line Output 9 and 10 (for Headphone Output 2).
This means that anything assigned to Line Output 7 in MixControl will come out of output 7 on the back of the unit, as well as the left headphone channel for Headphone Output 1. The same thing goes for Line Output 8, with output 8 on the back and the right headphone channel of Headphone Output 1. And the same thing applies for Line Output 9 and 10 with the left and right headphone channels of Headphone Output 2.
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