Passing values between two user exits - ABAP stack?

Is there something like an ABAP stack that I can use to pass data between two user exits? I need to hit a user exit on the billing header and one on the billing line item. I am getting data from KNVV so I would rather hit it once, in the header, then pass the values that I need on the line item via a stack of some sort. Is there a way to do this?

Regards,

Davis