A descendant tree means to only show descendants so I am not sure what you mean by showing people who are not descendants in a descendant tree. That sounds some new graphic display feature, but I am not clear the goal or how it would be constructed.
Now usually people's files are mostly connected. One solution (especially if you have large monitor or two monitors) is to open multiple family tree windows, and you can open any number in GEDitCOM II at the same time. In your example, do a tree for each of the two individuals you are interested in and resize them for viewing side-by-side at the same time. In other examples, you open enough to have everyone displayed someplace. I use that method sometimes to visuallze connections in my files.
John
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John Nairn
http://www.geditcom.com
Genealogy Software for the Mac