Multi-joins on table in universe cardinality error

I was getting errors when trying to join two fields in one table to two fields in another table… “Invalid expression. Parsing of expression for the business object’, etc… “has a different cardinality than the detected cardinality”.   I joined the fields–> tables externally by dragging the field name in one table to the other table.   This works in sql… but not in the data foundation.   What  I had to do to correct this was select on link externally table1.field1 to table2.field2, then right click to edit the join. Once in the editor click and control click the 2 fields in table 1 to select, the click and control click the 2 fields in table2 that you want to link to.  Once I did this I was able to “detect cardinality” and it worked.  I’m not longer getting “join” errors when I do the integrity check.