There are two predicates, so the expected cardinality is multiplied together

If the first is expected to affect 50% of the rows and the second 20% of the rows, then the estimate for the two together is 10% of the rows.

I go into a fair amount of detail on how in this presentation: http://www.sqlpass.org/24hours/2014/summitpreview/Sessions/SessionDetails.aspx?sid=7279