I entered 50% of T Slips (shared with a spouse) and my entries show up as two 25% splits on the S4-SUPP. Why?

You only need to enter the T-Slips which you are splitting with your spouse on one of your tax returns. You must enter the full amount that is shown on the slip, then specify the percentage that is split with your spouse. TurboTax will then enter the correct percentage on your spouse's return.