If you are using TurboTax Online, you'll have to start a new return in order to prepare your return together with your spouse.

If I have downloaded software can I join two tax files together to do a spousal return?

@Wheeler-Shaw Unfortunately, you can't; if you decide to prepare a joint return, you will have to start from scratch.

That's not entirely true - I took the one with the most entries and was able to add a spousal return to it by going to the View menu under 'personal information' and checking the box to add a spousal return.  Yes the second return had to be started over but it only had a few entries so it didn't take long.