I suggest that you don't do that, as the fact that the key value is missing is the indication that the record only contains data from one table, not both.
However, it is easy to create an expression to create a new variable to do exactly that. I tried it with the SASHELP.CLASS dataset, and when I create a new variable whose value is
(coalesce(t1.Name, t2.Name)) AS CombinedKey
it contains the complete set of keys.
Tom