SSRS : LookUp for two columns with "OrElse" ..Is it possible?

Hi ..

I have two datasets in my ssrs reports linked to two different datasources. 

datasource1: customer demographics

datasource2: product information.

All customers from product db MUST exists in Cust Demographics db but there is no unique column to link them. 

I need a report from product db along with customer demo. 

I am already doing the lookup based on calculated columns for two different fields (CustomerFullName+DOB) from both these datasources to match the information. There is one more overallping column "custNumber" but is optional. I want to macth the data with this number and if it doesnt exits then use the calculated field( CustFullName+DOB). 

I am successfully able to get data using calculatedField whic i named : (ForLookUp)

Getting error on this : 

=Lookup(Fields!ForLookUp.Value, Fields!ForLookUp.Value) OrElse Lookup(Fields!CustNumber.Value, Fields!CustNumber.Value),Fields!HomeCity.Value, "customer")

any suggestions?

Thanks!