There is no way to combine two separate tax returns. You have to pick one of them, change the filing status to married filing jointly, then add all of the other person's information. Or, if you want to keep the two separate returns for reference, start a new joint return from scratch.
If you are using the CD/Download TurboTax software you can save a copy of one of the separate returns with a different file name, then use that as the starting point for creating a joint return.
