I believe that is as far left as you can go. As I think you have found, the adjustment is in the Essential Graphics Panel, Edit and then "Set Horizontal Position" in the Align and Transform section. If the grid is already set to bottom left, you cannot set the horizontal position under zero. If the grid is set to center, if will only allow up to -144.
Using regular text, not captions, can be positioned further to the left.
Two active caption tracks are not allowed at this point. And nesting will not do the trick: only the uppermost nest will show its active caption track.