[OSM-newbies] Share nodes/ways between areas?

[David Carmean]

How do I share nodes/ways between two areas?  I have a park boundary and along 
part of it is an area of water.  It seems like I ought to be able to attach 
the new way segment for the water to two points along the original park border, and 
somehow designate the segment in between those two points as shared.  

I'm using JOSM and can't quite figure out how to do this.

Thanks.