Here you go.
So I blended the DS on Issue Number (well as they had the same name Tableau picked up the blend-field).
In DS1 I then created the following calc, referencing DS2
[In DS2 flag]
IF ISNULL(ATTR([DS2 (Blending DS Test)].[Issue Num])) THEN "No" ELSE "Yes" END
and I bring this field in, to get the result you want.
Check DS Dims through Blend.twbx 21.5 KB