Still not seeing where is your problem.
In your second form, you have a combo with months (?) and you are trying to store that selection with the id you get from the first form. And the problem is??? You don't know how to do which part?
From your first form, you past the id to the second form.
(when you call your second form)
SecondForm.EmpoyeId = selectedid
In your second form you have a variable like this
Public Employeid as int
Or long or whatever you need.
When you select from the combo (I don't know if you use a buttom or something else, but...)
first you validate the data with a sql like this.
SELECT COUNT(*) FROM table2 WHERE month = 'yourselectedmonth' AND employid = theidyougetfromfirstform.
If that count returns 1 then you are trying to insert something that already exists.
Are we going good now?