"a close friend of mine calls what i want a "unicorn". I dont know if thats the correct term or not."
It is, yes, when it comes to common poly terminology, in that you're seeking a new partner to share with your existing partner. Aside from being difficult to find (thus "unicorn"), this can present a number of problems. You've already identified one, which is that you may not be comfortable with them sharing love as well as sex, and in the end there is no way to stop that from happening if it's going to happen. Another potential problem is that she may develop a much stronger relationship with one of you than the other, which can lead to jealousy. What would you do, for instance, if you and she break up... would you insist that he and she break up as well?
We discussed these issues thoroughly on this thread, it may be useful to you: http://www.polyamory.com/forum/showthread.php?t=23779
I would also strongly recommend this resource: http://www.morethantwo.com/coupledating.html
If what you're seeking is fairness, rather than assuming that the best way to achieve this is for the two you to be involved with the same woman, consider the possibility of leaving the option open for him to find his own girlfriend. I get that this seems like it would cause more jealousy issues than "sharing" a girlfriend, but in fact it seems to cause far less in most cases.
Best of luck!