Input 1 mode should be stereo. If you do exactly what Chris did in the video it should work. Without a more thorough description or the preset file it's difficult to guess what steps you might be omitting/altering/adding.
1. 2 mixers in first column, shunts to complete those rows.
2. "BAL1" should be left in one mixer, right in the other mixer. (GAIN1/BAL1 are always the effective controls for input signal when a mixer is in the first column.)
3. Set mixer output modes to mono.
If you do that you should have each signal isolated to one row and hear both output regardless of monitoring setup and Output 1 mode--a blend of both signals will be in both Out1 channels.
1. 2 mixers in first column, shunts to complete those rows.
2. "BAL1" should be left in one mixer, right in the other mixer. (GAIN1/BAL1 are always the effective controls for input signal when a mixer is in the first column.)
3. Set mixer output modes to mono.
If you do that you should have each signal isolated to one row and hear both output regardless of monitoring setup and Output 1 mode--a blend of both signals will be in both Out1 channels.