To begin a new joint return on TurboTax:
You will not be able to merge two previous TT account to start your new joint return. You can import ONE of your 2015 returns into a new return, so choose the most complicated one. The other spouse’s information need to be entered manually.
When you enter your own information in Personal Info, you have to answer the question "Were you married?" If you click the button for Married, then a drop down will appear that asks, "Do you want to file this return together with your spouse?" Then you choose yes to file a joint return. You would then enter your spouse's information into Personal Info. Whenever you are entering income information there should then be a spot for you and for your spouse's income information.