I will have two students using Saxon 54 (or 6/5, 76, 87). Do I need to purchase two homeschool kits?

If you plan for the students to share the textbook, you can just get the Tests & Worksheets booklet for the second student. If you want them to work at the same time, you need the student textbook and the Tests & Worksheets booklet. However, it is usually cheaper to purchase two whole kits and sell the extra piece or pieces.