Generally speaking, CRON expression allows to specify minutes and hours of the job.
For minutes, it is possible to specify at which minute of an hour job should be run.
For hours, hours can be specified via enumeration (2,4,6) or via once per how many hours (*/2).
Taking that into account, following CRON expression can be used:
0 30 */2 * * ?
0 30 0,2,4,6,8,...