lookup confusions!!!!

Hi Friends,

Am new to BODS...i want to know the syntax for lookup function(not lookup_ext)  for comparing two column in lookup table...

for one column i could do that

syntax: lookup(TABLE_NAME,RESULT_COLUMN,DEFAULT VALUE, CACHE,COMPARE COLUMN, EXPRESSION)

condition :lookup(DEW_EnterpriseWarehouse_DS01.DBO.EMP, EMP_NO, -1, 'NO_CACHE', EMP_FIRST_NAME, SRC_EMP_FIRST_NAME) am getting the right output

But my question is how to compare two column in single lookup function.....

in the above example am using first name and taking the emp_id ...but i want to take emp id based on firstname and last name ....how to do that with the help of lookup function.....

Thanks.