you board both horses at a barn? tell me about the turnout. are they turned out together? who is dominant? how much time do you spend excersizing each of them?
Behavior changes like that usually don't come out of the blue. it likely has to do with his relationship with the new hrose and maybe some barn sourness and perhaps a lack of excersize.
Also, is it possible that you have not been as firm in your leadership with the first horse now that the second is here? Horses rise to the lowest level of the expectations that you ENFORCE, and no further. So , if you have not been consistently enforcing your expectations for good manners, then he is coming only that far in his obedience to good manners.