m-X Rename Buffer in two-window mode

[ZVONA%MIT-OZ@MIT-MC.ARPA]

Suppose I have a buffer FOO.  I use c-X 4 to put the buffer in both
windows.  Now do m-X Rename Buffer$BAR in one of the windows and do
c-X o.  You get a newly-consed empty buffer called FOO in the other
window.