This is already possible.

• Connect the first stereo pair to inputs 1-2, connect the the 2nd pair to inputs 3-4
• Use Insert routing to create a stereo insert. This insert will enable you to route inputs 3-4 to into the signal path
• Adjust the position of the insert to your liking. You can have it before all of the effects, in between, in parallel, etc.
• You may need to adjust the mix level to 50% depending on the insert routing.
• Please give this a shot after checking out the insert routing section of the manual and let me know if you need further help: https://cdn.eventideaudio.com/manuals/h90/1.11.4/content/edit-modes/routing.html#insert-routing