AddMixingTransform and RemoveMixingTransform not working as I wanted

OK, I have a person that I would like to combine different animations. I would like to make the person to walk but combined with a holdweapon animation that have animation only in the arms and in the rest of the body the walk animation. I have try so many things and try to figure it out how AddMixingTransform & RemoveMixingTransform works and AddMixingTransform works fine but I cant make it work as I want and the RemoveMixingTransform it doesn't works, it doesn't matter how I implemented I just always get and error when I run the game(I have Unity 3.4 so RemoveMixingTransform exists). Actually I want something like this:

 if(/*Player is walking(Input.Get....)*/){
 if(weaponinhand == true){
 //animation["walk"].is on all the body except for the arms
 //animation["holdweapon"].is for only the arms
 animation.CrossFade("walk");
 animation.CrossFade("holdweapon");
 }else{
 //animation["walk"].is for all the body
 animation.CrossFade("walk");
 }
 /*also I want that when I switch for having weapon or not
 it works because sometimes when I try it doesn't works*/

Thanks for your time :)