Implement a comparison feature. This would simply scan both panes, and compare by existance, date and size or content/md5.
After an item was found missing, a gost would appear in ITS place, when a conflict, the item would get some extra icons. Context menu on items would allow for actions. Additionally it would be great IF in Comparison mode, opening a subfolder would follow in both panes
Robert GID S. commented
Comparing 2 folders for missing and excess files on either side, based on time/date stamp or content CRC, would be a perfect enhancement to SE for easy syncing. Some feature like BeyondCompare in Windows.
Michael Glenn commented
This would be a really handy pro feature. I was about to suggest it myself. Sometimes you're not sure if two folders are in sync or you just want to see the differences.