R help - Mar 2009 - Find boundaries between two SpatialPolygonsDataFrame objects?

Dear R-helpers

I have two SpatialPolygonsDataFrame objects, but I want to plot only the
boundary between them. The boundary object is to be overlaid I plot that
I have made, so I would strongly prefer a method that does not rely on
grid graphics.

Thank you in advance for any pointers.

Kind regards,
Peter.