First I will say, being a unicorn hunter is a great gateway drug. I would never knock it for being an introduction to possibility.
However I think finding stable well adjusted people who want to be secondary to two people would be difficult. Thats the key. The couples believe combined they make up a primary but that just isn't the case. During the NRE phase this may seem Utopian, but how long can it last. How long until the "couple" becomes enthralled with themselves and their primary life. When does push come to shove
or... when does one of the couple change perspective on who is primary... and lose focus
To often was use triad to define a unicorn relationship. In reality its a scalene triangle. If everyone going in understands that and can live that in a healthy manner, then sure it can work. But how often can people slide in and our of primary and secondary roles. And the person getting downgraded is left insulted?
I do believe these can work, but... lets think of random thoughts
poly is rare
SINGLE poly people LOOKING for a couple are rare
successful long term poly relationships are rare
Combine all those and throw in bi-sexual into the mix...
Well, it becomes infinitely more difficult especially when you start tacking on months and then years.
so.. what can a couple offer... not much. Unless the unicorn has no interest in a primary relationship. Or... I suppose... if three single people fell into a triad at the same time. Then you could almost eliminate the tiering. But in an existing couple...that becomes difficult.