BE 2010 Duplicate Jobs

Running BE 2010.  I have a weekly B2D and a duplicate to tape right after it.  I would like to schedule a copy of the original B2D in order to copy it to a different disk sometime prior to the next B2D run the following week.  I am using "Duplicate Existing Backup Set" but am confused as to what to select from the "Selection list" pull down.  How do I know what duplicate backup set #### to use?

I initially used the same one the duplicate tape is using, but the following week the duplicate backup set #### changed to something else.

Am I overlooking something or not setting this up properly?  Any help would be greatly appreciated.

Thanks.

Seth