I'm not 100% sure what you're asking, but if you want to update a different table when something happens, you can use a Update Queries or just a currentdb.execute Update SQL statement

I have a single table for transactions.  When I add one It automatically adds the double entry record into the same table.  Its the next transaction ID. Same table.  If I change the account in the form that I created the double transaction, I would like to automatically change the account in the secondary transaction record.  Otherwise it will leave the transaction in wrong account and not show a transaction in the right account.

I do something similar, but I have an additional field that fills the opposing record's field with the the transaction number of the first one entered in the transaction. This way you can run an sql statement to locate the related trasaction records.