Hello ddoyen,
- Connect the guitar to In 1 and your modular synth to In 4.
- Create Insert 2 and set its Mix parameter to 50%.
- Position the insert so that it appears last in the chain after two Presets in series.
- Your guitar will be processed by both presets but the synth signal will only be mixed with the guitar to the output in stereo.
It would also be possible to connect the Iridium to Insert 1, which is I/O 3 on the H90. But because you’re using Insert 2 (mono) already the Iridium can only ever receive a mono signal; nevertheless, you can place it anywhere in the chain, even right before the synth. Note: if you place the Iridium between the Presets, the effect from the first Preset will be output in mono, but the signal from the second preset will be output in stereo.
I recommend this video to understand using the H90 as a mixer: https://youtu.be/0G_WtfUatSc?t=1190&si=FJH9Y9BgUlxhfyO9
