When existing VIs are opened, the front panel and diagram will be displayed at the positions they were in the last time they were saved. From one LabView sessions to the next, LabView doesn't keep track of the positions of VI windows unless they were saved.
You might be able to use the last-saved position to display a VI's front panel and diagram on two different monitors, but that also depends on the hardware and software that controls the use of multiple monitors. Some open all windows on one monitor, then you can move a window to another monitor. Do you have other applications that re-open on your second monitor?
You need to be careful saving VIs with their front panels or diagrams on the second monitor if you distribute your VIs to anyone without a second m
onitor: the window may just open off the screen making it hard to move back onto the screen when you can't see it to know where it is.