One Lease Two Terms Help

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I have two room mates who will be renewing their lease for a year. One of them will be staying for a year while the other one will be staying for 6 months only. They will be sharing the cost of rent for the first 6 months; once the room mate moves out the tenant who stays for the year will be paying the full amount until she finds another room mate. Once she finds a room mate I will do background check and have them signed a lease. My questions is what is the best way to have all this reflected on the lease? I don't want to give the tenant to sublease a new room mate with out a singed lease with me.