How do i add a second rental property?

Once you have completed all the entries for the income, expenses and assets for the first rental property the software will take you to the rental properties screen where you can add another rental property.

If you are using the online version of TTX, your screen won't look exactly like my screenshot below. But it will still have the "Add Another Rental or Royalty" button on it.

Once you finish with your first rental in it's entirety, you're returned to the Rental & Royalty Summary screen. At the bottom of that screen is a button for "Add Another Rental or Royalty"