Hi, and
There are a lot of couples out there looking for just another female to add to the mix, but to actually find a single bi female that you both click with is rare--hence the term "unicorn".
Because of this, many couples seek out couples with bi females, and put in their online ads that they are not interested in playing with the other male. Sometimes this works out, and sometimes not. Sometimes, the original couple finds a male half that they're comfortable playing with, too.
It basically all comes down to doing whatever you guys are interested in/comfortable with.
=)