First method:
*On 2nd Modal Page (the one with Data to be saved for the left Grid)*

    Type: Close Dialog
    Point: Processing
    When Button Pressed: Cancel

*On 1st Modal Page (with the left and right Grids)*

    When - Event: Dialog Closed
    Selection Type: Region
    Region: Left Grid

With the following True Action:

    Action: Refresh
    Selection Type: Region
    Region: Left Grid

Second method: Using a branch to go back to the previous Modal Page.

*Branch (on the 2nd Modal Page)*

    Point: After Processing
    Page: The previous modal page
    When button pressed: None (So that either cancel or save trigger it)