The average Arab is more sensitive than your average non-Arab, so you might be able to get away with carrying your hands one way with another horse, but not with these Arabs. Generally, excessive head-tossing means the horse is not happy with your hands. Get someone knowlegeable to watch you ride them to see if they can give you any pointers or to see if they notice you doing anything specific that is causing the head tossing.
Just an example: I went to ride today as a "job interview" to start training & showing for some people. They put me on an Arab mare, & she was an absolute doll. They told me that they always put people on her in order to assess their riding skills, because the mare will throw a fit (including tons of head tossing) if she feels at all uncomfortable with either the way she is being ridden or with the general "vibe" that the rider is giving off (nervous, aggressive, etc.)
I am of course not saying that you're a bad rider, there just might be something that needs a bit of tweaking in order to make these horses a bit happier so that the annoying head tossing will stop.