No. They can each sign a separate will.
It is, however, in our opinion better to have a joint will. With a joint will there is one document which sets out what happens if the husband passes away first, what happens if the wife passes away first and what happens if they both pass away together or shortly after each other.
Also, the joint will can provide for the massing of the estates should they want to leave a certain asset to, for example, one of the children after one of the spouses has passed away. A farmer might, for example, want to leave his farm to a son after he passes away.