Dragging Items Between Medialists

[verdigris]

I don't know whether this is specifically a Windows problem, but it might be so I decided to post here. Win7/64

I can open two instances of VLC;
I can assign the two instances to different audio devices;
I can drag mp3 files from a file manager into either instance.

So far so good.

What I want to do is use one instance for previewing through headphones, and then run the file "live" in the other instance. What I can't do, and would very much like to, is drag an item into the live instance from the preview instance's playlist.

Possible? How? I've tried every combination of left click or right click with shift ctrl or alt, I get the "+" cursor as if the file link will drop in to the playlist but nothing happens.

Thanks in anticipation.