put them into time out for 5 minutes and make them stay there, and when it is time for them to come out...you get to their level and talk to them face to face. and do this till they eventually stop

Boy may be boys, but they still need to come up to a certain standard of moral behavior. If it isn't going to be cute when they are 20, it isn't cute now. If they hit and bite others when they are 20, they will go to jail for assault and battery. At 20 months, probably the best thing would be to put them in separate rooms alone for a few minutes every time they do it. Tell them "no" firmly as a warning, then separate them. They won't want to be isolated, and after a while they will begin to respond to "no" without the separation. Kids are not stamped out with a machine. Every kid has a different personality. Yours may respond to time out, or they may not. Mine didn't. I didn't want to spank, but it was the only thing she responded to. However, whatever discipline you use, the goal should be to teach them the reason they must not do something, get them to stop doing it and remove the discipline as quickly as possible. If you are doing it right, you shouldn't have to keep doing it. It should also have clearly defined limits (you don't just keep hitting until they cry or until you feel better or as hard as you can with whatever is at hand). It had better be agreed upon between you and their mother as well. It should be planned beforehand when you are not angry and should be delivered without anger. The goal is NEVER to harm the child or scare them out of their wits. You should do only as much as it takes to be effective, and not one jot more.

This is an article I found for my sil and it worked very well....hope it works out for you. Verify and see what the root of the biting is and before they bite take the inocent child out of the way don’t put them into large groups if that’s where it happens. Take the biter somewhere quiet to calm down. When your child bites, use simple but firm words. Like NO. If you’re in a group, remove them from the situation. Explain that it hurts others and why you don’t like them doing it.When things have calmed down, try to help your child find a less painful way to express their feelings. This works well with children who are biting to try to show their affection. If your child’s expressing love, teach them to hug rather than bite whenever they feel strong emotions. If your child bites out of defense, show them how to tell somebody they don’t want him or her too close – to make the “stop” sign (a hand held up) – or even gently to push the other child’s shoulder – which won’t hurt but gives a clear message. Or teach them to come and find you instead if they’re angry. * Reduce the effectiveness: When children bite to gain attention, dealing with it is trickier. After the first big talking to, don’t try to continue to reason or explain. Give a firm “no”. Put your body between victim and biter and turn your back on the biter. Give the victim sympathy and the biter a clear message this is an unproductive way of getting attention. If time-out is one of your methods, now’s the time to use it. If the bite was over a toy or treat, remove it for a short while. If a child tries to control his or her mum by biting, try physically putting a part of your body in the way as they go to bite – an arm or a leg, which will stop them in their tracks. * Praise them for good behavior: Catch your child behaving well – not biting siblings, playing well in groups, not biting to get his or her way – and be generous with praise. Be specific – “good boy” becomes like water off a ducks back to them. Instead try: “how well you’re playing!” or “aren’t you kind and gentle to your little brother?”

Spare the rod and spoil the child. Better to control them now because when they get to school they will be trouble for you all the time. Make the kids mind or it is your fault if they turn out to be little heathens that everybody hates. Your responsibility to give them morales and dicipline.

Realise that this is not uncommon behaviour for this age, twins or not. They are taking out their frustrations on the nearest available object, which is with each other. They are coming up to the terrible twos, so don't just let it fester. Read all you can (both of you) about toddlers and how to manage (not punish) them. And try to find out if there is a twins' parents support group in your town - they can be very helpful. And please get over your belief that 'Boys will be boys'; this is a cop-out. Boys are children and children should not be allowed to be bullies, and it does not matter if their targets are siblings, friends or strangers. This attitude will just give them a good excuse not to develop self-control as they mature.