Information Steward rule bindings - Multiple Tables

Hi

Ive set up a connection between SAP ECC6 and IS 4.2 SP1.  I am trying to create a rule that will bind to both MARA and MARC in ECC6.

I set up the first two parameters to connect to fields in MARC, but when I try to connect the third parameter to a field in MARA, it tries to change the previous two to MARA also.

Am I trying to do the impossible?

How would I do this?

Please help

Michael