From what I understand, if H1 preset is set in PRE, H2 preset must be also in PRE mode, which means one cannot be in PRE and the other in POST routing. Am I right?

Not sure why you say that you can't have one in PRE and one in POST.

The main issue with having 2 units used in this way is when you have a parallel effects loop, as it is unclear how bypass should work. (See previous posts for discussion).