Not possible. When the H90 is in Dual Routing Mode, each path is independent and cannot be combined internally. You’ll need an external mixer.
Only option to achieve what you’re trying to do is described in the other thread linked above, where you use Insert Routing Mode and make use of the stereo insert.
Wow. I’m literally in awe. I must say, this is extremely disappointing. And there is not even any insert routing options for inputs 3 + 4. I should have purchased two H9s and saved myself the long backorder wait. Lame.
Also, they could have added two or four stereo tip/sleeve input jacks instead of four 1⁄4” mono tip/sleeve input jacks.