One: Be patient. There are approximately one berzillion books, songs, movies, comics, and whathaveyou about how hard it is for monogamous people to find partners.
It takes time and effort. The math is not on your side.
Two: People are not sex toys. People are people - you gotta treat them like it.
Three: Why are you doing this? Is it to offset jealousy on one side?
Because what you really want is a threesome? To try and explore non-monogamy without threatening your existing relationship? All of those require more self-work and honesty on your part.
Try to work out those issues together - adding a third will not magically solve existing problems. Four: What do you have to offer? Most requests for a third look just like that - requests.
You want to be a couple that other people are drawn to organically, not a couple on the prowl.