[Cialug] ssh tunnel between two cliens via the help of a server

[Claus]

Does anybody know if it's possible to have two ssh clients connect to 
the same server and establish a ssh tunnel to transmit traffic from one 
client to the other?

My application would be with VNC where both clients sit behind NAT and 
don't have public IPs.  Each client create a ssh tunnel to a ssh server 
and thus connect with each others.  Maybe with something like this:
  VNC server:
    ssh -R 6000:localhost:6000
  VNC client:
    ssh -L 6000:localhost:6000

Is that possible?
   Claus