Yes, you can include both tax returns in the same envelope when you send them to CRA. I do this all the time so that they are processed together.
This works the same way as when we e-file multiple years of returns together and want them processed together. Here, we e-file starting from the oldest return to the most recent return, and we select “Yes” in the checkbox on the Info Tab: “Are returns for prior years being filed at the same time as the 2020 return?”
Hope this helps.