Sorry if the question has been asked before.
If the H90 is setup as Dual Mode, is it (or will it be) possible to use INPUT 1 and 3 as two separate inputs and have both stereo outputs summed to OUTPUT 1 and 2 for example ? It seems not doable for now.
No this is not possible. In Dual Mode, Path 1 uses Inputs 1-2 > Outputs 1-2, Path 2 used Inputs 3-4 > Outputs > 3-4. The paths cannot be summed, Dual Mode is intended to create two separate signal paths.
You may be able to accomplish your idea using Insert mode. I/O1-2 is the main path, but you can use input 3 or 4 to insert another signal at different points in the main path using the routing page. The Return Level parameter for the insert will adjust the volume of that signal.