Waveshare 7inch HDMI LCD User Manual page 7

7inch HDMI LCD (B)
the second display (the one we want the touch controls)
6. Touch the center of touchscreen, to set this display as touchscreen. If there are
only two displays, after touch, the setting will finish, and you can find that
touchscreen controls itself now.
【Note】This method can be used to make touchscreens control different displays if
there are multiple touchscreen connected.