My requirement is to schedule a Job every 30 minutes. I can do this using two CRON Expresssions (as given below), but I want to see if we can do this in one expression ? Advantage of having one single job is that "if the job take more than 30 minutes to process, second job wont fire". By having two different expressions, they can be running together.

String cronStr1 = '0 00 * * * ?';
String cronStr2 = '0 30 * * * ?';