Follow mode is a minor mode that makes two windows, both showing the same buffer, scroll as a single tall virtual window. To use Follow mode, go to a frame with just one window, split it into two side-by-side windows using C-x 3, and then type M-x follow-mode. From then on, you can edit the buffer in either of the two windows, or scroll either one; the other window follows it.
In Follow mode, if you move point outside the portion visible in one window and into the portion visible in the other window, that selects the other window—again, treating the two as if they were parts of one large window.
To turn off Follow mode, type M-x follow-mode a second time.