Case Statement

I am using PL/SQL (Oracle) and I have a case statement that is dependent upon 1 field, but it drives the results of 2 other fields and possibly 3 fields in another table.  Here is the pseudo code:

case
  when mbr10.ssn is not null then mbr10.ssn as SSN, mbr10.eff_date as Eff_Date, mbr10.end_date as End_Date
  when mbr10.ssn is null and mbr20.ssn is not null then mbr20.ssn as SSN, mbr20.eff_date as Eff_Date, mbr20.end_date as End_Date
end

Can this be done and if yes what is the syntax?

Thanks,
Scott