First, I want to wish you luck on your journey. I completely get what you are saying and applaud the openess of you and your husband to find this other person. My husband and I have brought a third person into our marriage. We started out as friends, and we both ended up falling in love with her, and she loves the both of us. We have often compared it to the love we share for each other. In fact, he refers to both of us as his wives, and we both refer to him as our husband.
I am sure you have thought about this, but I warn you of the emotionality of what you are embarking on. It is a very tricky scenario to get around. There will be times that jealousy and such things can take you to the brink of disaster. Even with the love all of us share, it can be a slippery slope sometimes. But, as long as everyone is willing to sit down and talk about whatever issues creep up, and find a solution that works for everyone, then you should be fine.
Again, I applaud you and your husband and wish you the best of luck finding what you are looking for.