And if your two monitors are not the 'same" in terms of size, and resolution selected (System Prefs>Displays), you will not be able to freely extend from one monitor to the next.
If you can move your mouse from monitor to monitor, then you can drag and extend your timeline. Again, in System Prefs>Displays you set the left-to-right order of the monitors, and which monitor contains the icons of the hard drives (which becomes the default or start-up Monitor).
If the two monitors are not the "same" and you have your timeline at the bottom of the default monitor and can't extend it to the second monitor, then...
Try positioning the timeline more in the middle of your default monitor and then try extending to the second monitor. Ten reposition the timeline (vertically), until you reach the desired layout appearance on your set up.
Independent photographer, film maker and Producer. In the wonderful UK.