Providing the Teacher has allowed multiple file submissions on the Assignment, the Student can attach up to that maximum number.

If the Student clicks Save for Later after attaching 1 file and there is a maximum of 2 they can then go back in and submit another file.
User-added image

Once the Student has hit Submit the Assignment, the Student get the message that says "Are you sure that this is ready to be submitted? Once submitted no further edits can be made." from here the User can Cancel or proceed with Saving the Assignment.

If the Student has submitted the assignment and has only attached 1 file and there is a maximum of 2, the Teacher would have to allow a Resubmit in order to attach the other file.

To allow a Resubmit
  1. As the Teacher navigate to the Course
  2. Click Assignments
  3. Click the Assignment link for the Assignment you want to allow a Resubmit for
  4. Find the Student and click on the Students name
  5. Click Allow Resubmit
  6. Once the Allow Resubmit button is clicked the Teacher gets the message "You are about to allow this student to resubmit the assignment. Click 'OK' to allow resubmit or 'Cancel' to continue to show the assignment as submitted."