I use a 2 monitor setting (left/right) with one panel each. Each panel has a
separate set of items but both have a workplace switcher.

For both of these instances I use
Appearance: Miniature View
Number of Rows: 2

after logging out/in the first instance is double rowed O.K., but the second
instance is only single rowed.

for both plugins the file
~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml

contains
<property name="rows" type="uint" value="2"/>


While playing around I found out:

Whereas for the Workplace Switcher Properties "Appearance", "Show Workspace
Number" as well as for "Switch workspaces using the mouse wheel" a change of
the respective setting only modifies the chosen instance, a change in "Number
of rows" always modifies both (all?) Instances.


also, if I set

first instance: Appearance: Miniature View
second instance: Appearance: Buttons
=> Layout is preserved after logging out/in


first instance: Appearance: Buttons
second instance: Appearance: Buttons
=> Layout is preserved after logging out/in


first instance: Appearance: Buttons
second instance: Appearance: Miniature View
=> Layout is wrong (whereas first instance is double rowed O.K., second
instance is single rowed) after logging out/in

first instance: Appearance: Miniature View
second instance: Appearance: Miniature View
=> Layout is wrong (whereas first instance is double rowed O.K., second
instance is single rowed) after logging out/in


this is regardless of the panel's row size (my default is 36px, tried as well
80px)