To run a simultaneous release you need to provide a pubrelease.multi.yaml configuration file that describes each of the packages that need to be released.
The pubrelease.multi.yaml configuration file will normally be located in the tool directory of the main project but can be in any of the package tool directories.
The pubrelease.multi.yaml configuration file simply lists each of the related packages and their relative paths.
The order of the packages is important.
It is important that you place each package in the correct order for the release process to run successfully.
You should place the outermost packages first.
For example if you have the following pubspec.yaml files: