In my testing, I am finding that Xfce 4.12 no longer appears capable of working nicely with my Xorg Zaphod heads mode setup.  I do have the separate screens and desktops and panels for each, but when I launch an Xfce Terminal on Screen 1, it is opened on Screen 0.  Some apps launched on Screen 1 do display on Screen 1, but it just seems to depend on the app.  It's likely that I don't know (or forgot) all of the mechanisms involved with apps displaying on a given screen/desktop.

I've been using the Zaphod heads mode for a number of years with two monitors and two independent desktops (one for each with four workspaces on each desktop).  This is a workflow that is essential for the way I use this desktop computer.  I recall this has worked without issue since at least Debian 6 series through Devuan Jessie.

Does anyone have any ideas before I move this upstream?

I've no idea about any Zaphod other than B., but the display is usually set by the $DISPLAY variable, (e.g. DISPLAY=:0.0). With an X11 xinerama setup (one desktop over two displays), the alternative would most likely be ":0.1".

I've been testing the forthcoming Devuan 2.0 (ASCII) which features Xfce 4.12.  For years I have been using dual port graphics cards and dual monitor with Xorg configured for Zaphod.  Xfce up through 4.10 has worked very well in this configuration with two independent desktops with four works spaces each, dare I say rock solid.  With 4.12, I'm finding this is no longer the case.  For instance, on 4.12 when I try to open Xfce Terminal on Screen1, it opens on Screen0.  Various other apps (beyond those for Xfce) may do the same or open on Screen1 which is where I would like them to be.
Devuan ASCII has X.Org X Server 1.19.2 installed.  I am currently running Devuan Jessie which has X.Org X Server 1.16.4 and Xfce 4.10 and, as I stated earlier, has been flawless.