Are you saying you have a tableau dashboard which displays current month data
which you would like to compare to last month which is stored in a backup data store.
If this is the case you will need to add a new data source into current dashboard.
Add a filter on this new extract to pull in only previous month data.
Then you can blend the data together on a criteria where (current month) --> from current data =( previous month + 1 ) --> From Back up