Instead of doing several loads, unless specifically necessary, I'd load just once, renaming the date field, using field length for example
Table: Field1, Field2, If(Len(DateField1) > 0, DateField1, If(Len(DateField2) > 0, DateField2)) as DateField
depending on your data source, of course. Anyway, QlikView concatenates implicitly if all fields are named alike, so you can concatenate under the same table label as many sql/load statements as you want, provided they return always the same number of fields with the same name.
Nevertheless, you can use CONCATENATE keyword before the LOAD to force QlikView concatenate several sql/load statements, but you will have to take care on date field names, as you want all of them to link to the master calendar.
Hope this helps!