This is my first post and my new employment has brought me from a windows development environment to a hybrid Linux/windows development environment.

Question: How can I take three VNC instances ( :1, :2, and :3 )and configure them so that I can drag an open window from one to the other. [I am not sure if this is even doable, references to alternatives also appreciated]

What I am working with:

Windows 7 PC (multi-monitor)

Debian 8.1 using Xfce4 for VNC connections
(Note:Xfce4 can change to something else if that would make this work better for some reason)

I can create three VNC sessions on the Debian PC:
vncserver -geometry 1920x1080 :1
vncserver -geometry 1920x1080 :2
vncserver -geometry 1080x1920 :3

I can also open all of them using a VNC viewer on my windows 7 machine.

Problem: All of these sessions are independent of each other.

Requirements:
(1) Windows open can be moved from one screen to the next with out issue. (Does not need to be half in one screen and half in the other)

Note 1: Mouse can already move from one screen to the next with out issue since it is just three vnc connections to different displays (:1, :2, and :3) in full screen on my windows 7 environment.
Note 2: if there is something better than VNC that will do this, I am also open to those suggestions.

what I have looked into but have not figured out the "recipe" that satisfies my requirements:
xpra
synergy
xinerama
"clip" feature of x11vnc
some way to "detach" and "reattach" opened applications from one X session to another without having to shut them down and re-open them.

I have read some threads that discuss adding monitors using VNC with the physical graphics card, but this linux box is located away from humans and I would simply like to remote into it from home or some other remote location with multiple monitors, as needed.

thanks for you help!

I do not think that it would be possible. Each vnc connection is initiated with rfb password. This means you need to type the password of the user that is running the process.

This is where the issue comes in. If you have 2 seperate logins on the same machine with the same user, it is not possible to just drag and drop windows around from one user environment to the other. If all users could do that it would be a security issue and processes will become disorientated.

The best you can do is probably change the way vnc connects using something like xdmcp to provide a true multiuser login promt when first connecting.

But if you need to drag and drop and move windows, why don't you simply make use of the workspaces offered as a default function on most desktop environments?

The other option would be mapping a single session display to a 1:1 2:2 ratio as you have mentioned in your post, but as you also correctly mentioned, that will not allow interaction between different sessions.

Ericson007, thanks for the reply, I am not sure what "rfb password" means other than what you just said.

The box(es) I am working with are not in the same location and with "physical" video outputs disabled via bios with only one communication option for accessibility from an outside network (SSL). I tunnel the ports applicable for the VNC sessions I am using to my local machine and then open them up full screen on each monitor. I have been able to get 90% happier with this setup by running a copy past process tool that is integrated whenever a VNC server session is started.

I would be 95% happy if I was somehow able to transfer a GUI process from one session to the other with out having to shut it down on one side and re-open it on the other. I would be 100% happy if I could figure out a way to drag a GUI window from one display output to the next.

I am guessing it would have something to do with creating two "virtual" physical monitors and figuring out how to have one VNC server out one of those monitors and another VNC server output the other.

The inherent flaw in this method is that the VNC servers make two separate X servers, and X servers cannot interact in this way. The only way would be to make a VNC server with a geometry twice the width of one virtual "screen", and scroll sideways in your client. But that's not a real solution. However, I might point out that Windows couldn't have two separate servers either.