If you need to combine mulitple deductions into one place in box 12 you will need to use a formula. KB article 21588 How can I combine more than one tax, pay, deduction or fringe into a single box on a W-2? has information on how to create formulas to combine amounts

PR - Tools - Modify - W-2 Forms and on the amounts tab, you click each box (12a, 12b, etc) and enter a pay id, deduction id, fringe id or a formula.  Denise has given you the help if you need a formula.