Using Exam Wizard
To create an exam with more than one test, you can use exam wizard. Exam wizard can be opened from a link the Create Exam dialog. Wizard provides a step by step process for adding tests in the exam.
In the first step, you will be asked to provide the start date of the exam
In the second step, you can select one or more tests by clicking on Add Test link. You can also set start date and duration for each test. Duration specifies number of days/hours/minutes a test will be available for taking. The actual allowed time in test once started by a student would be same as test's duration specified while creating the test using test editor.