How do change my return and spouse’s separate returns to be done together as opposed to separate?

Unfortunately, once you have started your returns as two separate ones, there is no way to join them later. You would have to create a new return and specify that you want to prepare your returns together.

If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/replies/3456106