Hello Ian,
This is a situation with many answers and the best one will be determined by what you are trying to accomplish.
If you are just trying to open a second form to the current record of your first form, then open the second form's recordset (query or SQL string) and add a criteria to the primary key field to filter the recordset.
Reference the primary key field on the first form:
Forms!frm1stForm!txtYourPKID
IF you are trying (not sure from your question) to keep the records in 2 forms synchronized, then using the same recordset.
Leave the second form's recordset blank and on its OnOpen event type:
Me.recordset = Forms!frm1stForm.recordset (substitute your object names)
If this isn't what you are looking for, then come back with more specifics.
Cheers
Bill