In regards to your question:
I believe you are obviously using RAID at the Storage Level and then providing LUNS to the MediaAgent before creating the Filesystem (at the OS layer), technically there is no limt to how many Mount Path you would like to create from a Commvault Perspective.
However we would not recommend having multiple Copies pointing to the same backend Disk Storage Array (as this creates a single point of failure). So if your Storage Array goes offline, you lose your Primary and Secondary Copy. So by having different Copy pointing to different Storage, you eliminate the potential scenario where you might not be able to restore.
In regards to the SAS Tape Drive, the FAN-out configuration could definitely work as you are distributing the redundancy, in the off chance one MediaAgent goes offline, the other MediaAgent can still do the Backup/Aux Copy to Tape as it still has access. (Side Node - ensure the firmware of your Tape Library is able to handle the SCSI reservation with the configuration that you are seeking)