If you can afford to keep a mistress, and this woman is ok in that role, and you feel that this woman, can handle it.
'good outcome' is subjective. However, it's possible. I believe that it's more successful in European culture, than it is in American culture.

I didn't go down the other woman path, and am glad I didn't. However, I was at a point in my life, where I really, felt that it's all that was available.

Let your mistress, know, there is support, just difficult to find.