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There are two different multi-build concepts in colcon. The first one is how many executors to use; that essentially limits how many packages colcon will attempt to keep in flight at once, and is controlled by --executor-sequential/parallel. The second one is how many cores each package will use to do the build. That is actually controlled by the underlying CMake implementation (and Make below that). If you want to ensure that only a single C++ file from a single package is built at once, you'll need to make sure both of them are set properly. The following should do what you are looking for:

MAKEFLAGS="-j1 -l1" colcon build --symlink-install --merge-install --executor-sequential