How long have you had the horses? How old are they? I would separate the one who is headshy and won't allow you to halter him. If you can get a halter on I would leave it (make sure it has a breakaway), then take the time to gradually gain trust (give treats, just hangout with him, hang around when you feed him), brush him, specially the areas he likes. Then work you way towards the head softly. As he relaxes gently hold halter for a moment, pat him. As he learns to relax and tolerate this you can then gently put a halter Over the one already on him (which gives you some control). Repetitions till he no longer resists. Lots of treats for each positive. This may take considerable time, but well worth it.