Yes, you would need to use Dual Routing for this.
Yes, you should set Inputs/Outputs 3/4 to line level if they are in your amp’s FX Loop. Inputs/Outputs 1-2 should remain at instrument level.
If you put both of the Presets on Path 2, then all of the H90 FX will be in the FX Loop, and Path 1 will act as a Thru path. You can have Path 2 be in Series or Parallel, see below:
Once you have all this connected, you just need to go to the Routing menu to adjust wether the presets are on Path 1 or Path 2.
Also please note that when you switch from Insert Mode to Dual Mode you will not see the same Programs you have created since they have different routing. The easiest way to handle this is to export your Program list in H90 Control, change the routing mode, and the reimport the list once you are in Dual Mode. You will be prompted to convert the list to Dual Routing on import and then you can use your same Programs in Dual Mode.
