Adjusting color balance when cloning in iMovie

Hi, in iMovie I'm using Video overlay > Green/Blue Screen to create a clone effect.

As this is an exterior sequence the light in the two shots was slightly different so I'm also using the colour/light adjustments to try to match the two shots.

However... as a person in dark clothes walks into the frame during the second shot, the colour/light adjustments automatically re-adjust to compensate for the change in this causes. In other words the colour balance adjustments don't stay consistent throughout the second shot. The end result being that after the auto adjustment the line between the two overlayed shots is obvious and spoils the effect.

Can anyone help me set the adjustments so they don't change to compensate?