First question, the second table will contain only the timestamp?
Second question, does your form display multiple records from the master table or a single record?
If you only need the timestamp in the second table, and you have only a single record displayed on your form, then my solution would be best, although I would just make a button for them to click that would enter a new record into the second table, which if linked by timestamp will automatically enter the timestamp into the second table.
On my test form, I display one record from the master table at a time and a blank field from the secondary table (in this case a dummy field named tested). I named the dummy field from the secondary table as tested. Then the button has the code:
tested = "X"
test.postrecord()
Since the timestamp is the key field in the secondary table, it is automatically entered. I couldn't get this to work with just the timestamp field, I needed at least one other field in the secondary table that got a new value.