Hello,
You can definitely do this. However, you will have to start the return over.
At the top of the screen, go to Tools > Return Manager > Create a new return.
Then, at the start of the return, you'll get to a screen that says: Who are you preparing this return for? Select My spouse/partner and me.
Enter your names.
On the What's your marital status screen, select Living common-law.
On the How do you want to prepare your return? screen, select Yes, I want to prepare our returns at the same time.