There may be some flexibility available to you, depending on your employer. A different start date can be negotiated if you are interested in starting sooner than two weeks (or later), have a long-term employment contract, or would like to take time off before starting a new job.
In general, it is best to express a willingness to get started as soon as possible. It will make the transition to the new role easier for you and the employer, and it will help ensure a smooth transition.
You should be discreet with your answer if you have another job while applying for a new one. Such questions can prove to be a test of your ethics.