One tenant wants to move out

[Alex Clift]

I'm about to renew a year lease with three long standing tenants. They're great tenants. One of them asked to only sign for 6 months as he's planning to move by then. They've offered to find a replacement tenant (obviously vetted by me). I'm ok with this but should I put something in the lease that states the other two are responsible for the full rent when the third moves out?