Disney Answer: Yes
Disneyland was opened in 1955. When Disney decided to open a second theme park at Disneyland, they built Disney California Adventure in the parking lot located right in front of the Disneyland entrance. Once California Adventure finally opened in 2001, it created a great pavilion located between the two parks. From one entrance to the other, the two parks are less than 200 feet apart, so you are just a minute or two to cross the pavilion and enter the other theme park.
While the Disneyland monorail passes through both Disneyland and Disney California Adventure, there is not a monorail stop in California Adventure (there is one in Tomorrowland in Disneyland and one in Downtown Disney closer to the Disneyland Hotel). You cannot use the monorail to hop between the two theme parks.