Let's say on the left one you want to access some slow, unreliable media (i.e. network share). When doing that, the whole application blocks, becoming useless.
Another example: if you calculate the size of each folder on one panel, the whole application is blocked, until that gets finished.
My suggestion is to separate the panels, in a multi-threaded/processes way, so that if a panel is frozen, the other one is still usable.
Or even better, even if the left panel is frozen, the user can open a new tab on the left panel

We see this trend in Chrome, where each tab has its process.