Problem

vCenter does not allow users to combine both ioMemory modules (DIMMs) of an ioDrive Duo into a single VMFS datastore.

Cause

vCenter does not allow this task.

Resolution

To perform this task, connect directly to the ESX/ESXi host with the vSphere Client.

With the Host object selected -> Configuration -> Storage -> <Datastore to expand> -> Properties -> Increase -> <select the 2nd DIMM to add to the datastore>.

More information on this operation can be found in the VMWare KB Article: Unable to grow or expand a VMFS volume or datastore

While extending a datastore across devices may be needed to increase space, it is not a RAID configuration, and it does not add performance (as in a RAID0 array) or redundancy (as in a RAID1 array). It is a concatenation (SPAN/JBOD) of devices.