Consider situation:

You have opened two Nautlis windows ( folders ). Nautlis window A ( folder A ) and folder B.

Now you want to send folder B into folder A so the Nautlius window (folder A) would contain two tabs - folder A and folder B. That means I should be able to "drag and drop" folder B into folder A, but I cannot.

I suggest that there should be some kind of dialog allowing to choose, what would be target for drag and drop operation, for example, when you drop the dragged folder, you have popup message dialog, looking something like this:
 ________________________________________________
|                                                |
|"Please choose the resulting effect of operation|
|                                                |
|      o Drop as new Tab in <target folder>      |
|      o Drop as Moved content from <src folder> |
|                                                |
|________________________________________________|