I have a new garbage disposal. When it runs, water comes up in the other sink. I have looked at your websites and read your book, but have not found the answer.
Installation could be an issue here. Ideally, the disposal and the other sink basin will have a separate trap that is connected with a “Y” into the drain line in the wall.
In some cases, a disposal will be installed with a “T” to the tail piece of the adjacent sink (see illustration). With this installation, water can be pushed up into the adjacent sink. You can buy a special “T” for this connection to direct the water down, but it’s still a questionable installation.