Many SS officers also held reserve commissions in the Wehrmacht. However, the division of authority was no different than any other military. While officers and NCO's may have a form of general military authority based upon their rank, what you wouldn't see is an SS officer hi-jacking command of a Wehrmacht unit - at least not without the consent of the Wehrmacht officer.

There were times, espcially on the Eastern Front, when Waffen SS units were attached to Wehrmacht Army Groups. This was no different than when the Luftwaffe Ground Units, and eventually even the Kriegsmarine sent sailors to the front, found themselves under the administrative control of the Army. Typically this was because one commander was directed that he was subordinate to another, and not because a member of a different military branch claimed control.