You can call anyone any title that you'd like..or even have them both walk in together as your moh's and the most important females in your life...there are no rules..do what makes you happy..its your day! and maybe have your daughter in law wear an almost identical dress to your friend..make them both feel amazingly special..because having a friend that said the things she said to you is one in a million, and typically women are more likely to tell you drop your man, you are missing out on all the nights, and everything imaginable to take away from your happiness...so when you find the one genuine sincere friend, you are right to want to cherish them..im saying this from the opposite spectrum of having been the one that was happy for everyone, have spent nights talking to friends, having their bf's actually cry to me that they were driving them crazy, drove to hospitals at the crack of dawn when they gave birth, and have even driven some of my friends mothers to doctors when they werent able to. Now it's my time..and some of those same people arent coming to our wedding..so my point in saying all of this is if you have a true friend then you make her feel appreciated.
They can and should both be important and who says you have to be traditional by having one moh?