Trampling over each others pasture is the main source of irritation, and constant irritation over time can drive one mad. These pairs can have very similar interests but they go about them very differently; one partner would tend to think that the other has the right objectives but is approaching them in the wrong fashion. It's very difficult for these partners to divide responsibilities or define separate roles such that one will not eventually try to pee in the other's soup; normally, a third party has to do it for them. These relationships seem to work when both partners essentially live separate lives, coming together only when mutual support is needed (you fight me, you fight both of us); when pastures aren't even adjacent, even contrary comments are far less likely.......

a.k.a. I/O