The file displays should stay the same proportions when the viewer pane opens. Maybe they aren't really starting off equal, and it's just harder to see when they're quite large.
Try resetting them to 50/50 by double-clicking the slider between the two file displays.
The (proportional) size of both the file displays and the viewer pane will be saved as part of the default lister, and Layouts and Styles more generally. You can use that to define the sizes that things should normally be.
Depending on your settings under Preferences / Launching Opus / Default Lister, the default lister may be updated when you close a window, or you may need to use Settings > Set as Default Lister to update it explicitly.
Layouts and Styles always need to be updated (loaded, adjusted, then re-saved) explicitly if you want to change the sizes in them.
You can also use commands to resize the panels to specific sizes of percentages on demand, or when you open panels. Scripts can react to panels opening and resize things based on whatever logic you want, if you want to customize things in a lot of detail (although that should not be needed).