Internal Auditor

First, each organization has the authority to define the competency requirements of its internal auditors. 

Second, as good practice internal auditors should comply with two basic requirements:

They know the standard;
They know good auditing practices
 

So, your internal auditors for ISO 9001:2015 already know good auditing practices. Do they know ISO 13485:2016? Can you certify their knowledge?

As long as they meet those two criteria, they can be your internal auditors.

The following material will provide you information about internal audits:

- Five main steps in the ISO 13485:2016 internal audit - https://advisera.com/13485academy/knowledgebase/five-main-steps-in-the-iso-134852016-internal-audit/
- Free webinar - How to perform an ISO 14001:2015 internal audit - https://advisera.com/14001academy/webinar/how-to-perform-an-iso-14001-2015-internal-audit-free-webinar/
- free online training ISO 9001:2015 Internal Auditor Course - https://advisera.com/training/iso-9001-internal-auditor-course/ />- Book - ISO Internal Audit: A Plain English Guide - https://advisera.com/books/iso-internal-audit-plain-english-guide/