Some do both. Most cyclists agree that to be successful, the rider must focus on one or the other. You do only one thing. Road and mountain bike racing are as different as night and day. The two types of cycling require different techniques and training.
Road racing is different from mountain biking because the riders work more as a team.
Mountain biking tends to be more individual. Teamwork vital Teamwork is vital to road racing. Each team has four members. One technique riders use to control the race is drafting, when one cyclist follows closely behind his or her teammate. This allows the second rider to rest for a portion of the race because of reduced wind resistance.