I can't tell from your description whether you really need a JOIN or if you need a UNION.
I think you need to show us a specific example of what you are after.
Regardless of whether you need a JOIN or UNION, how will you tell MySQL which one of the pair to pick?
In other words if I have
-- Table1 --
117 greenish pink
-- Table2 --
116 yellowish mauve
What do you want for the final result? For userids 116, 118, and 119 it is easy. Only one table has userids 116 and 119. And both have the same frogwarts
value for userid 188.
But what do we show for 117? greenish pink
How do we tell the computer which one to pick?