i've been thinking about this...
a dual couple is all about reaching a balance in terms of function strength by helping out the other person by covering their weaknesses and receiving the same treatment in return. however, this leaves both persons relatively un-dualized in terms of function values. in fact, the relation will likely reinforce any biases and comfort-zone constricting dogmas that may exist in both persons on this front.
a super-ego relation is relatively tolerable in terms of compatibility and absence of conflict and potentially comes with many of the strength balancing benefits of a duality relation. in addition it DOES bring balance and moderation in terms of function values.
of course there is one major pitfall: super-egos don't balance each other on the introversion/extroversion front, nor on that of rationality/irrationality.