The Naming Series for 2 Doctypes are synchronized

In a System I’m working on for my University, The Transactions DocType and The Transactions Actions Doctype use different naming series, but they are synchronized. That is to say if I create a Transaction and it’s given 0001, if I then create a Transaction Action it will be given 0002 not 0001, and then the next Transaction will be given 0003 instead of 0002 and so on. How is it possible for this to happen and how do I disable it?