What is the plural of ‘the last will and testament’?
The plural would be formed the same way it usually is: with an -s on the nouns. Example:
These are the last wills and testaments of those who died in the accident.
In legalese it would be simply "last will and testament of Jane Doe and John Doe, dated on such and such date" if they are joint subjects contained in the last will. If they each have their own last will it would be "the last will and testament of both Jane Doe, dated on such and such date and John Doe, dated on such and such date.".