No, this is not possible. If you only connect out of Output 1 the H90 automatically functions in mono, even if you were to use the stereo insert last in the chain.
If you will not be combining two algorithms per program, the workaround is using Dual Routing Mode and spitting your signal before the H90. Feed one to Path 1 (mono) and the other to Path 2 (stereo). Use the same algorithm for both.
