H90 outputs 1-2 are a stereo pair. If only 1 of these outputs goes to another signal processor / effects pedal, then you will not have a true stereo signal.
This is not recommended for stereo operation, but there is no harm that can be done to your pedal if you try this, so you can see if you like how it works this way.
There is only so much you can do with a given setup. Typically, a user will either choose stereo OR wet/dry. It is not possible to do both without reconfiguring your setup.
You could try a Wet/Dry/Wet setup if you’d like wet/dry and having it be in stereo: https://cdn.eventideaudio.com/manuals/h90/1.9.7/content/setup/wet-dry-wet.html
