My toddler and my four year old have this problem sometimes. Here's what I do. Set a timer, and tell them that for the next 10 minutes, they MUST scream as loud as they can. They are not allowed to stop screaming until the timer goes off. When the timer goes off double check to see if they have any screaming left they feel they must, and then tell them that since they have got all their screams out, there will be no more screaming for the rest of the day. Give them three warnings for further screaming, and then give time outs. You may also want to try seperating them for awhile and having them play quietly by themselves until they calm down.