Can I use multiple versions of the same inspection plan at the same time?

No. At any point of time, you can only use one version of a plan to do inspections.

When you create a new version, you specify a start date. If you have not specified an end date for the previous plan version, the previous version becomes obsolete a day before the new version's start date. From the new version's start date, you can start using it for new inspections.