Rational Dual Relationship: Living for each other.
Irrational Dual Relationship: Living with each other.
This is a very tentative hypothesis based on some dislocated sampling and my observations may or may not be generalisable in the form that I've produced. However, it seems to me that that rational duals tend to have a greater sense of commitment to the relationship, whilst irrational duals have a commitment to each other, but less so the institution of a 'together'. Rationals therefore are more likely to become 'fused' whilst Irrationals are more likely to maintain their individualities.
For example, my parents are in a Rational (semi-dual) relationship. My father had to give up his studies half way through his Phd because my mother wanted a house due to very cramped housing living with my grandparents (housing in communist China - not fun). He of course wishes he had had the opportunity to complete his Phd, but he doesn't resent my mother for this, partly because of reasons related to his character (personal responsibility ftw!) but also partly because it was the relationship that demanded this, not my mother that demanded this. Interests/investment is shared rather than belonging to any one individual.
Reading the thread on 'IEEs and the problem of other people' made me think about this as well. Change that is motivated by internal, personal desires and does not take the relationship into account can be seen as a form of betrayal by a rational partner. Since they restructure their interests so that they 'belong' to the pair, rather then the singularity, having the other partner pursue interests that belong to their 'self' rather than the pair essentially rebuffs the premise of the relationship. It's not about how rational duals 'stifle' each other, but rather how change is negotiated between the pair and interests are pooled so that the other person can also 'partake' of this interest/change/motivation.