Yes, you can assign a candidate to as many jobs as you want. To assign candidates to additional jobs, go to the candidate's record and under the "Job Assignments" section on the upper right of the page open the action menu (three dots) and select Assign to Job.
You'll then be asked to select from the list of open jobs. You can only assign candidates to open jobs, if you want to assign a candidate to a closed or filled job, you'll need to re-open it first, which can be done from the job record.
You can also assign candidates to jobs from the action menu when doing research on the Candidate Manager from the "by Job" "Search" and "Unassigned" tabs.