This study addresses the two parallel machines problem, where machines are flexibly maintained within a given interval and the makespan is used as a performance measure. The maintenance interval is set in advance and the maintenance is attended by a single server which can perform one maintenance at a time. The maintenance times for both machines do not exceed the maintenance interval. The problem is strongly NP-hard. Four case, unequal or equal maintenance times combined with separate and continuous maintenance operations between two machines, are considered. Optimal solution for there cases are proposed and size complexity and computer solution times are provided to demonstrate the models'efficiency.