If I am primary POA for my parents, can I step aside for the secondary POA?

You can resign your POA any time you like, and then the secondary POA will take over as per your parents' instructions.

What you can't do, or I don't think so anyway, but it might be worth checking, is "suspend" yourself and come back later. So are you sure you want to do this? Do you mind my asking what the reasons are?

Only if you feel you are unable to perform the duties of being Power of Attorney. Could you work as a team with the secondary POA?