As this is a highly requested feature, we're working on updates to allow for teachers to co-manage sections, but at this time we don't allow merging of accounts or sharing of sections between more than one teacher account. We would recommend just changing the credentials of one teacher account overseeing all students to something that is easily shared among many parties.
That being said, note that students can be joined to infinitely many sections at once. If you have a co-teacher that you want to also be able to view student progress for, just have that co-teacher make their own section, then have all students join their section as well (while remaining in your section). You as the teacher can also copy students into other sections - see the guide on moving students for this process. Through either of these methods, both you and the co-teacher will be able to see all student progress, in any course.