Two Plans or One Plan with multiple plan elements

Currently we have only PPO plan in our company.  So we have setup a single plan in benefits and there are plan elements in each of those plans.

Now we want to offer high-deductible (HDHP) HSA-eligible health insurance also.  Does this be a new Plan on its own.  If so how to make sure employees are not selecting both the Plans during Open Enrollment in Self Service.

If it need to be Plan element on existing Plan then how to make sure they are not selecting both in open enrollment.