There are currently no plans for this. Also, while RawTherapee is by no means ‘slow’ software, rendering two outputs of possibly different zoom-levels and high resolution, is presumably going to be an unpleasantly slow experience…
Edit: well, what do you know, there is still an open issue about something similar on the bug tracker Detachable floating toolbox · Issue #4775 · Beep6581/RawTherapee · GitHub