Be careful with your preparation. Your answer sheets, style and value of test items should be identical for each exam to keep on track. You don't want a student to say that the other exam was easier. A goal in future could be to prepare more scrambled exams per unit. Even though you have tried to eliminate cheating by using different exams, you will still need to look for crib notes.
Students are very inventive regarding cheating.
We have all discovered that. You will not be able to catch them all over the year, but make it a goal to eliminate as much cheating as possible. A third unit final should be in your unit folders. This seems like quite a bit of work but eventually the extra mile will pay off. Here is the reason for this third tough exam.