Spring setter injection is one step, but I have to declare an instance of the manager in the java class, of which the package must be available. Otherwise the manager type cannot be resolved. One possible solution is to make the manager a 3rd-party library, then deploy it to the server so that it's available to both plugins.
For the dependency issue, you might tweak the pom.xml.
There is an instance where something similar to this was done: Lib folder for plugins.
Moving the Manager into a separate jar is a good solution, because you are isolating the Dependency, so that it can be used in multiple plugins. Also, it is easy to do this!
I found an alternative solution. What I didn't try is installing the manager as a maven dependency by configuring the pom.xml. It might be possible to install the .jar of the other plugin like so:
Create a directory in your plugin project called lib and copy the other plugin .jar and store it here.
add dependency to pom.xml:
Then open command prompt if running Windows at the project directory and execute:
mvn install:install-file -Dfile=./lib/TheOtherPlugin.jar -DgroupId=the.other.plugin -DartifactId=the-other-plugin -Dversion=1.0.0 -Dpackaging=jar
Then you can try importing class files from that .jar.
Let me know if you manage to get it to work.