SSH -r

Assuming two people can both SSH into box A.


x -----ssh-------->. A --------------- ssh ------------> Y.


1) User X :

 ssh -r 19999:localhost:22 -I

2) Then user Y: ssh's into A, and runs 'ssh -p 19999 X@localhost`.

Now user Y just SSH'd into user X's box, proxying through the A box.