I got married in May of 2018 and plan to file a joint return. Can I import two TurboTax files from 2017, one for myself and one for my fiance in 2017? If yes, how?

[xmasbaby0]

No.  

You will not be able to merge two previous TT account to start your new joint return.  You can transfer ONE of your 2017 returns into a new return, so choose the most complicated one.  The other spouse’s information needs 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.

Best Wishes!

[fanfare]

take care to use the account whose name you want to appear first on your tax return.