Kizzie - I'm sure it's harder with twins. My two are different ages and sizes, so the little one tends to get hurt very quickly, so it is quite clearly unacceptable.
Anyway, in terms of tactics, we just have zero tolerance of any violent or aggressive behaviour ... all the standard methods: rewards, bribes, sticker charts, time-outs, withdrawal of privileges, etc.
Also until very recently we would ALWAYS got out first thing in the morning for some physical exercise - a bit like walking the dog, really. It doesn't seem quite so necessary now, though.
We also don't have any (toy) weapons in the house (guns or swords) and anything that gets used as such goes straight in the bin.
It really winds me up when people say "they're just being boys, it's normal" - school don't tolerate this sort of behaviour, so why should I?