we dont remove synthetic keys always, but its depends on scenario.
and we dont remove synthetic key by doing concatenation always, there are other many ways to remove synthetic keys.
let say you have ID, Name columnS in customer table, and ID, Name columns in supplier table,
so qlikview will link these two columns automatically,and will produce a synthetic key, but its not right link, so in this case you need to rename columns in 1 of tables to remove the synthetic key....
make a search about when to remove synthetic keys on community, and how to remove , there are lot of discussion and documents, and you will get the clear concept about...
Read this linked document...
Hope it help...
