Form and Subforms

I have a form with two subforms:
Subform A= a query that matches expense records from Quickbooks to a more general category.  This is a datasheet form
Subform B= a form on the original table for the expense records; this is a regular form, one record at a time

I have two separate ones because I want the users to see the general category name, but also to be able to edit the records in the original table (I've added a field where they an assign an account)

My question is:
Once I've got the two subforms on a form, how do I link them so that scrolling through the datasheet pulls up the same record on the other subform, where they can add notes and assign an account?  They share the field "txnlineID"