The ability for taskview to only have multiple desktops on the primary monitor is actually a requested feature I've repeatedly seen. So if this is ever fixed in Display Fusion, it would be nice for it actually to be a toggle.

https://www.reddit.com/r/Windows10/comments/3f5l0z/task_view_on_multimonitor_configuration/
https://www.reddit.com/r/Windows10/comments/328xvq/seperate_virtual_desktops_over_different_monitors/