Hi @dbogacz -
While items that your co-teacher have already graded won't show in your To Do List, you can still access them through Speedgrader. You can get to this by looking at the right-hand side of the screen while looking at the details for an Assignment, Quiz, or Discussion. You could alternatively look at Grades and open the gradebook to pinpoint your task. Then, in the Grades Summary Card find the shortcut to Speedgrader as well.
I co-taught a class a few years ago, and we enjoyed being able to provide comments on all student work. Students really liked being able to receive feedback/feedforward from TWO teachers for some items. It's quite a neat experience for everyone.
I hope this helps! Let me know if you would like more details or if you have any follow-up questions.