AIST0002-User Exit - Validation of user defined field

I have implemented the enhancement aist0002. I added a user defined field in a user defined subcreen and added it to the leasing tabstrip. I would like to validate this field against a another field in sap's leasing subscreen. In the pai of the user defined subsrceen the field of the other subscreen does not get filled. Is there a way to access the field of the other subscreen on the leasing tabstrip from the pai module of the user defined subscreen?

Thank you

Tom