Configurable duplex system structures, like the IBM 3084, are capable of operating as one single system (single image mode) or as two independent systems (partitioned mode). The single-image mode of operation is more tolerant of hardware failures because of component redundancies, but on operating system failure all processors are lost. In the partitioned mode, the outage of any hardware component or operating system brings down the affected half of the duplex system. The problem is to select the mode of operation. Since multiple performance levels are involved, a composite performance reliability measure like performability is required. On the basis of the hardware and software failure rates areas are demarcated where one of the operation modes is better than the other. The sensitivity of the choice of operation mode to the goals of the installation is demonstrated. The use of a steady-state performability measure instead of transient performability is shown to distort the choice of operation mode. Hence, simple-to-compute tight upper and lower bounds for the hard-to-compute transient performability are derived.