The short answer is...you can't...or at least, I've never heard of having two or more of the exact same app installed on my iPhone. Any time there are new updates to any of my apps, I go to the App Store, and it downloads the new updates. I've never seen an option to allow me to install another instance of the same app on my phone.
So, to answer your question...when you are using the Canvas Teacher app on your iPhone, you'll want to tap on the three-lined hamburger menu icon in the upper left corner. Then, select "Change User". This will allow you to add another login (via "Find my school" or the "QR Login" options), or you can return to your current login listed under the "Previous Logins" area. This Guide has a bit more detail for you to look at:
Hope this helps a bit. Sing out if you have any questions...thanks!