Countif with 2 specific criteria

I am trying to find a formula, possibly an array, that will allow me to calculate a total only when 2 criteria are present. Please see attached.

In the attached file I have placed what would be 5 separate sheet of data onto one sheet for ease. Sheet 1 to 4 show a student name and who assesed them during an exam.
What i am trying to do is to get sheet 5 to look up each sheet and tell me how many times a specific assessor has sat with a specific students. Sheet 5 shows the results I am trying to achieve, but I have put them in manually. Is there any function in Excel that will allow a Countif to occur but only when 2 adjacent cells show the exact data required, E.G only count if Student aa sat with Assessor z.

Hope this makes sense when you see the file attached.

Many thanks to anyone who can give guidance.

place this in B13 and drag across and down,

Formula:

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