I have had little luck with the setup. Doing it exactly like the
manual says does give very good results. The preamps are never
separated and always seem to run simultaneously, regardless of
settings (although which is most prevalent DOES changes depending on
the FX loop control).
just an idea: some algorithms have stereo, some have mono inputs. when
you use a stereo algorithm, but have connected only one return, you´ll get both
preamps at the same time. you could try to connect both returns via a split
cable and look (no, hear!), what´s happening with mono and stereo algorithms (if
you haven´t done that already).
best regards
klaus