When I played around in creative in Minecraft 16w05b I seed an issue with dual wielding animations. Here will be a video wich you need to see before you know what is going on. First look at the iron axe and then look at the wood planks. When I place wooden planks the axe moves that means when I place the block with left hand right hand moves. Please watch the video on this link to see the issue: https://www.youtube.com/watch?v=kptqdwMSzFM