With join you are most likely always creating a new table so you would have to also handle that. Other option is to use Update instead
Names Default To Here(1);
dt = New Table("Untitled",
Add Rows(3),
Compress File When Saved(1),
New Column("A1", Numeric, "Continuous", Format("Best", 12), Set Values([1, 2, 3]))
);
dt1 = New Table("Untitled 2",
Add Rows(3),
Compress File When Saved(1),
New Column("B1", Numeric, "Continuous", Format("Best", 12), Set Values([4, 5, 6]))
);
wait(1);
dt << Update(
With(Data Table("Untitled 2")),
Replace Columns in Main Table(None)
);
Close(dt1, no save);
(try this also first with the interactive option)
-Jarmo