Please note that JMIR reviewers are typically acknowledged at the end of the published paper (in case of acceptance and publication, otherwise they stay anonymous).
So the way to address this depends on who wants to be acknowledged:
a) if only you want to be acknowledged and the other reviewer doesn't want to be acknowledged (please confirm with them), no action is required. Simply fill in the review form. In the comments for the editor, you should disclose who wrote what part of the review. However, this will not be acknowledged in the manuscript
b) if only the other reviewer wants to be acknowledged, please go to the review form and enter the name/email address of the other reviewer under "suggested reviewers", and decline the review request. In addition, you may file a ticket with email@example.com requesting the other reviewer to be assigned (state the four/five digit ms ID number and the fact that you entered the other reviewer as a suggestion). An editorial assistant will then assign the suggested reviewer
c) if both you and the other reviewer want to be acknowledged, please proceed as above (go to the review form and enter the name/email address of the other reviewer under "suggested reviewers"). Do not decline the review request but accept it. Also file a ticket with firstname.lastname@example.org requesting the other reviewer to be assigned. An editorial assistant will then assign the suggested reviewer. The student (co-reviewer) and supervisor should submit the same peer review rating (e.g. "B") and reviewer feedback (copied and pasted from the supervisor's review) and be sure to add the following sentences preceding the student's review that reads: "This is a co-review with another reviewer on this paper. The content will be identical in both reviews". In addition, disclose that you worked with your supervisor to complete this review in the confidential notes to the editor.
Please note that options a or b are preferred.