What I understand is that you only want to have a monthly backup and retain it for 2 months
The endorsement of the first of January (1st Full)
The endorsement of February 1 (2nd Full)
When running the March 1 backup, requires the January 1 backup to be purged
If the answer to the previous example is YES,
your retention must be 1 cycle 60 days
If the example does not correspond to your need, it would be useful to help us specify your request with similar examples.