I just wrote on another thread and will now go and copy/paste... as I will be repeating myself.
Really, in my belief, if something is not working then try something else... maybe go find your own partners and see how that goes.
Starting with what you said...
What you seek is near to impossible to find, yet it seems to land on peoples doorstep sometimes when they least expect it. Usually without looking.
It's almost impossible to find someone that will love both of you equally and whom you will love equally. Sex, sure, no problem, even at the beginning during NRE the going is good until the person, that is the cause of your relationship structure to become a triad, realizes they prefer one over the other, usually the male. Or they feel like a third wheel or are really a secondary in the whole arrangement.
There is a ton of potential for failure in unicorn hunting... why? because usually the original couple has not grasped the fact that they need to stop acting as a unit in the search and start being independent.
It's kind of a dichotomy. The couple seems to find they are interested in building their relationship and so want to add a woman. The idea is to fulfil what they are missing in some way and that she will do this FOR them. The woman of course is to be fulfilled also as that is only fair, but when the chips are down, she will usually be secondary and therefore not achieve the same rights as the couple to love, caring, closeness, consideration etc.
So, its important for the couple to be independent in the relationship that develops, yet how can they do that if they don't act independently in their search and in their lives ahead of time...how can they come together when it actually happens. It is likely not going to be as is thought when there is a whole lot of independent minded people all in it together, rather than a whole bunch of co-dependent people... if that makes sense... co-dependent triads are a very rare find.
My suggestion for this? Be independent and find your own paths in life and love while walking side by side... if you should be lucky enough to have a unicorn fall on your lap then you will be glad you did... if not, work on a vee relationship dynamic and be happy with it. There is nothing wrong with a good ol' fashion vee!
I also suggest doing a tag search for "unicorns" and/or "secondary" "seocndaries" so as to see what others have written before you. There is a really good thread called "a secondarie's bill of rights" that might help... found by tag search for "secondaries"