I\'m working on a project which shall contain two targets. One for building a Framework to use in Mac development and another to create a static library to use in iPhone dev
Probably the easiest (in terms of what you have to do) way of doing it is two projects (which you can wrap up as sub-projects of a dummy umbrella project if you like) which use the same sets of sources—one targeting Mac OS X, the other iPhone OS.
Alternatively, can you not create two separate targets within the same project, and alter the “Base SDK” from the “Build” tab of the “Get Info” window for each? I don't have Xcode in front of me to check, but I think it'll let you do this.
(Of course, with either approach, you can add a third target: a static library for the simulator, which will be useful when testing!)