Service accounts are administrator level functions, usually, so I, or another admin, changes the password if it's needed. So we never store them for that reason.
I also never share them. If you ever have issues and need to reset or change the SQL Server Agent account, do you want to bounce SQL Server to give the service a new password? Bounce 2 other unrelated SQL Servers? If it's never happened to you, good for you, but it makes for a bad day.
Or having an instance restart and not log on because the password has changed.
I would never share service accounts across instances or any other service. Maybe SQL Server DB and related services on the same instance, but that's it.