SOURCE=both doesn't mean anything so Opus will probably just that at best.
I think I understand what you want now, and the basic Set DUAL=toggle,remember is as close as I can get.
I would send GPSoft a feature request asking that DUAL=remember be extended to remember which side things were on as well as which tabs/dirs were open. Right now, AFAIK, you can only prevent it from swapping the sides over by always closing the right-hand side (which the SOURCE=Left made happen).