I want to do our tax returns together but started mine and then his separately. I cant get back to the page that allows me the option again

Due to the complexity of calculations related to marital status, once you've moved past this section, you can't go back and edit it. If you've already moved past this section, you'll have to start a new return.

Here's how you start a new TurboTax Free return: https://turbotax.community.intuit.ca/replies/2965552