How do I add secondary role responsibilities into my resume?

I'm a manager with a substantial main role in service delivery which I've been doing for several years.  I've been asked to lead an internal operational team as a secondary duty.  This would be considered a completely separate role and it could have been filled by a new hire.

Do I put both on my resume separately?  Or list them as concurrent?  Would I start a new section from the time I took on the second role or would I just add it as a responsibility under the first role?

for example.  Imagine you are a warehouse shift manager and you are asked to run the Lean Six Sigma team as a secondary role.  How would you put the two together?

Thanks for your thoughts