In dual mode, the H90 functions as two separate effects processors. In a normal pre/post setup, I/O 1&2 would be connect to the front of your amp for pre effects and I/O 3&4 would be connected to the amp’s effects loop. From your diagram, it looks like the PRE path is correct but I do not understand how Path 2 (Post effects) are being connected. It looks like you’re trying to use insert mode rather than dual mode.
Can you explain what you are trying to do? Maybe I can recommend a better signal flow.