I would write two separate documents.
One a "Early lease termination" or whatever, saying that (tenant name) will be paying $X (the amount of which is the april and may rents added together) for ending their lease early , and their lease will end March 31.
The other will be a new lease with the other tenant that ends May 31.
Does that make sense?
Also, I should add: Consult a lawyer, don't listen to a random person on the internet. ;) But that's what I would do.