I'm not aware of a direct way, but an indirect way is to choose the "Match Flag" option when creating the join, then you can manually/programmatically delete any rows that have the label "Both" in the "Match Flag" column. I've put a script below that does this as an example.
dt1 = open ("$SAMPLE_DATA/Hot Dogs.jmp");
dt2 = open ("$SAMPLE_DATA/Hot Dogs2.jmp");
dt1 << Delete rows (1); //to make an outer join possible
dt2 << delete rows (2); //to make an outer join possible
dtjoined = dt1 << Join(
With( dt2 ),
Match Flag( 1 ), //<-use this later
By Matching Columns( :Product Name = :Product Name ),
Drop multiples( 0, 0 ),
Include Nonmatches( 1, 1 ),
Preserve main table order( 1 ),
Output Table( "joined" )
);
dtjoined << Select Where (:Match Flag == 3); //JMP uses column values for the text
dtjoined << delete rows ();