Ariesbabu -
It will not overwrite tapes present in the same partition ( no matter how many slots a partition may contain)
That would depend on the OPP of the media set and the Date/Time that each tape was closed
MBGS
To guarantee what tape is written to, yes you would need to create a partition for each job, but you can come very close by loading the tapes into the partition, then re-labeling the tapes in the order that you want them to be used, and verifying from Tools\Options\Media Management that you have selected "Overwrite reusable media in the Target Media set before scratch media"