When I've had relationship trouble, the trouble is really a function not of the relationship but of the two people in it. The relationship is the interaction between the two people. It cannot be other than a function of its parts. The question is the relationship between the parts and the (dys!)function produced.
Some people (not many) have successful containment fields, which means that the personal difficulties do not creep into the individual's manifestation in the relationship.
At the most basic level, the problems in the relationship are represented by the sum of the problems in the individual people.
There can be neutralising effects, too; compensation results in reduction: 4 + -2 = 2.
The most pernicious arithmetic operation is multiplication. If person B's problem interacts with person A's problem, the problem in the relationship is the multiplication of two: 4 x 4 = 16, not 4 + 4 = 8. If this then collides with another of person A's problems, 16 x 4 = 64. The progression can be geometric, as an interactive loop is created.
I've never successfully salvaged a relationship once contempt and cruelty have arisen.
The relationship problems I've solved have been solved only through taking the Steps and removing my digits from the arithmetic. Note that 0 x 4 = 0.
If two people are unwell, I'm unclear how a healthy relationship can be produced. Containment fields can contain, but, if the individual problems are pervasive and everything is contained, there is no relationship. I don't know how any of that could work.
Don P would say, 'I don't know how to have a healthy sick relationship.'