[My thanks to Heath Raftery for validating some of my thinking]
I have actually made this scenario work, without requiring everyone to
set their personal XCode preferences to change the build location.
The key to solving the problem was realizing that the built products
location can be set per _project_ (formerly, I was aware only of this
setting in Preferences, and I didn't think it was The Right Thing to
require all of my colleagues to change theirs just to make this one
project work). So, by rooting both projects' builds at the same place,
the dependent project was able to locate the build products of the
precedent project, and now everything is happy. Er, the dependent
project builds (building the precedent project if needed).
Still, I don't think it's such a stretch to think that the dependent
project could have grokked the precedent project better, so that it
could go grab the build products wherever that project happened to