Dock moves to either bottom of main screen OR left of second screen - can't do left of main screen
The Dock will only go to the left of the left-most monitor, or the right of the fight-most, [based on your setup in System Prefs > Displays > Arrangement &amp; not on their physical placement if this does not match.]

It will not sit in the middle.
If you want it to go to your current working display, it has to be set to the bottom, &amp; you need 'Displays have separate Spaces' switched on.