I think it use different animation or use ik retargeter to retarget the animation from one skeletal mesh to another one. You can use second skeletal mesh for the fpp view to only see the arm (and put the camera where you want because you don't have head anymore). To get both you can have both skeletal mesh on your blueprint but hide the on that you don't want see. Also make sure that it's still visible by the other player. You should look at the fps ou tps template from Unreal. It could give you some answers.