Resize both windows so that they can be shown side by side.
System Settings → Workspace → Window Management → Window Placement
Click on the Advanced tab.
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Window Placement: change to Smart
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Tick the box labeled “Allow apps to remember the positions of their own windows, if they support it.”
You can do this by creating a special window rule for Okular.
System Settings → Workspace → Window Management → Window Rules
However, I strongly suspect that Okular will not accept that, because it was not designed with that type of usage in mind.
By the way, you can also use the window rules to force an application to always show its windows on the same monitor.