Using same field from different tables

[#1]

Hi

We are pulling in Salesforce Lead and Contact data into Qlikview but essentially we want to report on this as 1 table. For example we want to know the total number of leads and contacts created in a particular month. I have am using the expression:

=Count (distinct ContactId) + Count (DISTINCT LeadId)

in a graph but what dimension can I use to have it ordered by created date. I have tried a dimension with the expression:

=if(isnull(monthname(ContactCreatedDate)), monthname(LeadCreatedDate), monthname(ContactCreatedDate))

But when I drill down on the chart by created month it only pulls those records with both fields populated; LeadCreatedDate & ContactCreatedDate (i.e. converted leads).

Is there a way to combine 2 fields from seperate tables or should this be done in the Load script?

Thanks

Jon