Hi Jim,
I was in an Workflow training last week and I had a similar question. Eric Beavers created the following workflow and it worked!
In my case I had an AFKS with an invoice#, and multiple Serial# (AFKS1)
I also had an AFKS with Serial# and Item# Item Decription. (AFKS2)
I had a ocument with Invoice# and a MIKG of Serial# Item# and Itemdescription.
First of all, in your AFKS allow multiple Autofill Keywordsets, Your primary key will have multiple entries, the first keyword of MIKG, which is a secondary index will be different per entry.
In your workflow assign at least these 2 tasks:
Keyset - Set Keyword Value from Autofill Value (AFKS1) value=Serial#
Key- Autofill keyword set on this document
-with option Replace secondary values when only one instance exists.
Assign the workflow to the document and it will fill automatically all documents.
Credits for Eric Beavers please 
