I would guess the preschool is on the verge of being sued by the parent whose kid was hit by a block and needed stitches. Tell that to the director, if he or she has not decided to take any action up to this point. Your school does owe it to the other children not to have them be targets.
Well, the problem is that if you quit, the next teacher will probably also quit. But that is not as bad as the fact that very soon, I would guess that the school will start losing pupils because of these kids. The director of the school needs to know this and make some decisions.
Timeouts at there age should only be one to two minutes anyway. Sounds like they should be in a play area of there own and away from other kids. I am not sure that a child under two could really be called mean as I am not sure they are aware of that concept. Sounds like they play very rough. Which is probably the way they play at home. If so getting them to play together nicely would carry over to school. I wonder how supervised they are at home - twins at this age can be very tiring to the parents. Which is all the more reason for the parents to not want their kids kicked out of school. All in all if parents and school can work together - that is a win/win situation.
To change their behavior, its the constant immediate consequence course of action. Tough part is that it has to be constant - which takes work. Probably will take about two to three weeks to be effective. And they can't be allowed to even do it to themselves as that reinforces the idea.
There is a book called, "Hands are not for Hitting," http://www.amazon.com/Hands-Hitting-Ages-Best-Behavior/dp/1575420775
And while it is aimed at the 4 to 7 crowd, reviewers have used it successfully (it sounds like) for as young as two. Its not very expensive ($6/7) through Amazon (used is even cheaper). You might try it, and then give it to the parents.
One thing for sure, you can not do this by yourself. You need either the directors help or the parents.
Hope this helps.