What flight planner are you using? I would second the use of Little NavMap for flight planning and that app can simply be dragged onto the two monitors, wherever you want it .
If you’re using the built-in MSFS flight planner (World Map), you are out of luck. I don’t believe you can move that by itself.
The MSFS map, where you can view your flight plan, can be popped-out, as mentioned above, by clicking the center button (the box with an arrow pointing to the upper right-hand corner of the box) of the VFR Map window. You can then drag the map to wherever you want on the two monitors.
Hope that helps!