My question: Can clients be member of two clusters? 

For instance, we have three hyperV hosts in a HyperV-cluster in Commvault - Can they also be a member of a Windows cluster at the same time?

I tried adding the three HyperV hosts to a Windows cluster in Commvault, but it fails:

Cluster group test] configuration was saved on CommServe successfully.
Remote nodes update failed: (Use force sync option to update the nodes later.)
>Server1]. Reason: <There was error during initialization of information from input XML / local registry.>
>Server2]. Reason: <There was error during initialization of information from input XML / local registry.>
>Server3]. Reason: <There was error during initialization of information from input XML / local registry.>

I am thinking this might be by design, and they are not supposed to be member of multiple clusters..?