I want them to get inserted in the signal path at the places where inserts 1 and 2 are represented on the on-screen diagram, however that’s not what happens. For example when I set both inserts at the beginning of the signal path, only one of the two stereo signals gets inserted (the 3/4 one).
Insert 1 and Insert 2 are simply names for loops. They do not correspond to channel numbers. As per the Quick Reference guide and the user’s manual, Insert 1 and 2 are each mono loops that are connected to I/O 3 for Insert 1 and I/O 4 for Insert 2.
I also tried turning to Stereo Insert mode, but then only inputs 3/4 get fed into the signal path even though it shows 1-2 on the screen. This seems like a bug, but the manual seems to indicate this is an intended behavior? Why does it not show 3/4 if it’s actually inputs 1/2 that are being inserted to the signal path at that spot?
As stated above, Insert 1&2 is simply a name that indicates mono insert loops 1 and 2 are paired together to create a stereo loop. But the connections are still I/O 3&4 on the actual pedal.
Is the problem that the H90 is incapable of summing both stereo inputs and feeding them in at the same spot?
Eventide never intentionally set out to make the H90 act like a mixer for both sets of stereo paths, but maybe through experimentation you can make adjustments to the Stereo Insert Mix levels to make it behave in such a way. If you are using Insert Routing Mode, try connecting one synth to I/O 1&2 and the other synth to I/O 3&4. Create a Stereo Insert which will show up on the display as Insert 1&2. Setting the Insert Mix level from zero to 50% might work. I have not tried this. Try it and report back.
Other than running Dual Mode is there no way to use two stereo inputs simultaneously and change how they are routed into the effects?
Dual Mode is made for this because the H90 behaves like two independent stereo effects processors that have nothing to do with each other. You’re free to use one algorithm per side or both algorithms on one side in series or parallel configurations per Program.