An attempt at super-ego modelling.
In all super-ego relationships (romantic ones), the relationship tends to fall apart due to the dual-seeking function being the partner's 7th function, that is, it takes some strain to keep it active.
But how is it kept going for a whille?
- EJs and EPs: they are quick to start, and quick to collapse, due to the common extraversion.
- IJs and IPs: slow to start and slow to collapse, due to common introversion. Drifting into mutual boredom and sapping of energy.
In all cases, the identical temperament, together with the perceived differences, is a source of attraction.
Apart from the common temperament, the 6th function - 8th function axis is what keeps it going: it provides the common interests, but at a superficial and non-essential level.
-INTj-ISFj and INFj - ISTj : common ground in Si and Ni
Such couples will spend a large portion of their time at home enjoying good food and drink, or going out for that purpose. The Ne IJs will provide some input as to the opportunity of actions of the Se IJs during more serious moments.
- ENTj-ESFj and ENFj-ESTj: common ground in Ne and Se
Such couples will spend a large portion of their time going out to do interesting, fun things of short duration - like movies, theater, plays, concerts and the like. In more serious moments, they will discuss political power plays, at work or with acquaintances, offering mutual help.
- ENTp-ESFp and ENFp-ESTp: common ground in Te and Fe
Such couples will spend a lot of time in fun social activities with other people in a larger group; in longer conversations, the Ti EPs will provide information, and answer questions, in matters of interest to the Fi EPs.
- ISFp-INTp and INFp - ISTp: common ground in Ti and Fi
For the sake of the Te IPs, the Fe IPs will spend a lot more time with them on one-to-one activities than their inclination dictates, since they'd prefer to have activities in a larger group. In turn the Te IPs will have to get used to providing clear-cut Ti input to the Fe IPs' questions when asking for decisions. This is perhaps (but I may be biased) the most "inertial" of all superego relationships, held together by sheer lack of activity and with both essentially spending time together without ever "being" together.
I suggest that IP-IP superego are those that are the most difficult to start, but also the most difficult to break up once they have reached the stage of living together etc.