How should schools balance subject expertise and fair substitute workloads?

I’m building SchoolFlow Agent to help school leaders in Saudi Arabia find suitable substitutes when teachers are absent.

Suppose two teachers are available. One teaches the same subject but has already covered several classes this week. The other teaches a different subject and has covered fewer classes. Who should SchoolFlow recommend first?

Would your answer change if the class needs someone to teach the lesson rather than supervise students? If you’ve handled this in a school, I’d love to hear what rule you use in practice.