following the vSphere 5.5 Configuration Maximums, in a single cluster you can only have 1 VSAN datastore, so what you need is not supported right know, you can have multiple disk groups in a cluster but only one datastore.
What you can do its generate multiple FTT levels to tolerate failures.
Here are the link of the Configuration Maximus, and a capture of the relevant info.
Hope this info works for you.
I think you misunderstood the VSAN concept.
VSAN takes local disks and a local flash devices, and aggregates those resources of at a minimum 3 hosts in to a single shared datastore. Any availability or performance requirements are to be specified through the policy, and you assign that policy to a VM. Through that policy your VM will be then provisioned in such a way that it will meet the requirements.
When you design a VSAN cluster it is highly recommended to have uniform systems and uniform disks. So all disks preferably the same type / size.