Comment 43 for bug 190744

I think I'm with you. Let me try to write what I understand. On a user system there is foo (in package foo-1), which links to libfoo.so.1 (in package libfoo1-1).
Now we have a new libfoo with a different soname, and so we have libfoo.so.2 in package libfoo2-1 and foo in package foo-2 which links against libfoo.so.2
If the user upgrade foo-1 to foo-2, obviously it will also upgrade to libfoo.so.2 since libfoo2-1 will be a depends of foo-2, so, everything is fine, no dummy is needed (well, libfoo1-1 will still be on that system but in a more complex case I can understand why you would want that).
If the user installs libfoo2-1 then foo will still be ok since that will not remove libfoo1-1.

Wish somebody could write this stuff on a wiki page somewhere....