Since these two Datastores are on a RAID 1 setup, I thought I would be able to create additional Datastores by using the free remaining unpartitioned space of the same 2x 1TB HDD.
So if I understood correct initially the partition should be set to the same size of the RAID 1 setup. Then multiple partitions can be created from the partition free space each holding a different Datastore.
So potentially I can install another temp HDD and create a Datastore on it, move the current Datastores to it until I remove, re-create the full partition on the RAID 1 so then I copy all back.