Anything that requires splitting a signal is not possible in Dual mode. I misunderstood your intention with Dual mode thinking you wanted to use one Path (IN 1/Out 1) for one amp and Path 2 (In 3/Out 3) for a second amp’s effects loop.
If you wish to split signals with the H90, this is only possible using Insert routing mode. Create an Insert and set the Insert Mix to 0%. May I suggest this video on splitting signals for Dry/Wet rigs? Perhaps it will give you an idea: https://youtu.be/0G_WtfUatSc?feature=shared&t=786
