If the rent has not been paid in full for the month you can serve the notice of non-payment of rent (Form N4) but make sure both names are on the notice. This notice gives the tenants fourteen days to pay the outstanding rent and, if not paid within that time, then you can apply to the Landlord and Tenant Board for an eviction order. If one of them moves out in the meantime you can still proceed to evict the remaining tenant if he/she cannot pay the rent.