As has been said elsewhere, single bisexual females who want to have an exclusive romantic relationship with both members of an established couple are exceedingly rare, and are in tremendous demand. That's not just in Florida, that is everywhere, based on what I have seen on poly fora over the years.
So any couple that is searching for this is going to have to work really hard at finding, and interesting a prospective partner - they are not out there simply waiting for the next couple to come along - they can afford to be extremely picky because they have such a large choice.
I have been trying to help these folsk in the poly ads to try to show more about themselves that might make a suitable candidate have interest, because so many are surprised that an ad worded like "We are a loving couple looking for a lady to join our marriage" (i.e. essentially the same wording that everyone else puts out there) doesn't get available bisexual females falling over each other to reply...
Like any sales or marketing task, you have to know your market and your position in your market. So many are totally unaware of that, and react with extreme hostility when that reality is brought to their attention.
"Listen, or your tongue will make you deaf." - Native American Proverb