M.,
Is the 20 month old hitting or biting during or soon after a loud activity? Is this happening after he loses an argument about a toy or just after coming in from playing outside?
Children at this age have needs and impulses they cannot control and the adults in their lives need to teach them this control. But first the adults need to see the pattern to help them find another outlet for their behavior. It can be difficult to find the pattern when caring for many children.
If you see the child needs to hit redirect him to a pillow (if appropriate), for biting a tooth brush or soft plastic toy can help sooth his biting need. Talk with his parents to brainstorm ideas and if nothing seems to be working you may need to restrain him for a count of 10 (like the other mom posted).
When my son was 15 or 18 months old he hit his head on his bedroom wall out of anger. I tried different ways to stop the behavior, but nothing seemed to work. Finally, I restrained him on my lap until he calmed down, it took about 15 minutes. Then I explained to him that I would restrain him again if he continued his behavior, he disliked being controlled that way so much he never hit his head on the wall again.
Finally, I think the little boy should learn the ASL sign for sorry if he can't talk or he should verbalize if he can and indicate to the person he hurt that he is sorry. At this age he will not mean it, but like with adults learning a new skill, sometimes you have to fake it till you make it. Requiring children to apologize to the people they hurt makes them aware of the feelings the people around them.
http://www.signingsavvy.com/sign/SORRY
Good luck!
-C..