The H9 has a Pre/Post routing configuration that allows you to place certain presets on the “Pre” path and others on the “Post” path. You can find this setting in H9 Control by going to Pedal Settings > Routing. However, Pre/Post is limited to 2 mono paths because the H9 only has 2 inputs/outputs, so you would not be able to use stereo effects in the FX loop.
If you have both input and output jacks connected on the H9, the H9 will always be used in a stereo operation. If you are only processing on input/output 1, you’ll be missing the other half of the stereo image.
The H90 can achieve a mono path for the Pre path and a stereo path for the Post path, but the H9 is not as flexible for this sort of thing.