This applies to the Scarlett 18i20 and Saffire Pro 40
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. The same thing applies for Line Output 9 and 10 with the left and right headphone channels of Headphone Output 2.