The primary data set is specified by the CKPT1= parameter on the CKPTDEF initialization statement and the duplex (secondary) data set is specified by the CKPT2= parameter on that same statement:

    CKPTDEF CKPT1=(DSN=dsname,VOL=volser,INUSE=YES),
            CKPT2=(DSN=dsname,VOL=volser,INUSE=YES),
            MODE=DUPLEX,DUPLEX=ON

There is no requirement that these two data sets be on unique volumes; however, for performance reasons, and increased recovery potential, it is strongly recommended that you place them on separate volumes.