Ideally, what you want to do with PRE/POST effects is more easily done using Dual Routing mode. But because you want to split the output of the H90 this can only be accomplished using Insert Routing Mode.
Connect your Iridium to Insert 1. Place Algorithm A before Insert 1 for the “pre” effect. Place algorithm B after Insert 1 for your “post” effect. Place Insert 2 after algorithm B but turn the Insert Mix down to zero. The level to your amp is controlled by the Send level of the insert and the stereo mix from the H90 will go on to your interface.