Don't anchor them to the left. Keep them underneath the centered anchor, and position / resize them yourself based on screen width. It will be easier.
Alternatively, anchor one panel to 0.16667 left (33% of the screen, centered from the left so divide by 2), second to 0.6667 (2/3rd of the screen, centered from the right so 1.0 - 0.3333), and set the first panel's clip width to 1/3rd of the screen, second to 2/3rd of the screen.