I've never used qmake with Xcode and subprojects, only cmake. But maybe this could help

The 'subdirs' template

The 'subdirs' template tells qmake to generate a makefile that will go into the specified subdirectories and generate a makefile for the project file in the directory and call make on it.

The only system variable that is recognised for this template is the SUBDIRS variable. This variable contains a list of all the subdirectories that contain project files to be processed. It is essential that the project file in the sub directory has the same name as the subdirectory, so that qmake can find it. For example, if the subdirectory is called 'myapp' then the project file in that directory should be called myapp.pro in that directory.