when you create a config that will be reused you must have exactly the same items in both packages. whether they are variables, connection managers, etc.....

build the config in one package then use it in the second by adding and selecting "Reuse Existing"

You can't edit the config in one package then use it in the other it will fail.

If you want to add a connection manager in one package and use configs for it you must add it in the second package but of course you don't need to use it, just make sure it has the same name.

One technique that I use is to build every package with set of default connection managers one for logging operation one for calling shared procedures, and I like to have a generic one that I can wire up later if needed.

Then I use a global shared config for all of them. I also use a trigger on the SQL table to prevent updates.