Right-click on the desktop, select Display settings
Scroll down to the bottom of the display settings. Under "multiple displays", ensure Extend these displays is selected.
Test again by moving your mouse from one display to the other. If you are still not able to move your mouse from one display to the other, continue to the next step.
Scroll back to the top. Click and hold one of the displays and move it to the other side. Then, click Apply.
Test again by moving your mouse from one display to the other.