First I would suggest going out for coffee or dinner. All three of you. Give a basic intro-nothing about sex/relationships/expectations.
Just sit down and talk about mutual interests they may have (you should have an idea there since you've dated both) and see if it's possible to have a nice dinner together just hanging out and talking.
Hopefully if it goes smoothly one of them can take the ball and make plans for the three of you or the two of them to go do something that allows for getting to know one another better.
IF that all goes well then the conversation should naturally turn more serious as the friendship blossoms and that would be a good time to talk about interests and concerns of all three of you in making it work, like how to deal with jealousy. Lot of links on here about dealing with jealousy-do a search, read read read, then talk talk talk about it.
Makes a huge difference.
"Love As Thou Wilt"