Might be worth running it past your SE, but given the requirements, I would think a HyperScale X 2300 Appliance in each site would be more than sufficient. 

Agree however how are offsite aux copies handled to the other site? Hyperscale X integrates with Cloud but we don’t have that and would only have another system at the other datacentre.

Given the relatively small volume of data, I would just point a Secondary Copy to the alternate site HyperScale Storage Pool.

Ok so this storage pool is available to a totally separate instance of Hyperscale X? How is it presented to the other instance?  Sorry but I have zero knowledge of Hyperscale X. 

The appliances operate independently of each other. 

I’m oversimplifying it, but at the Commvault level, a HyperScale appliance is really just a collection of MediaAgents and an associated Storage Pool. They’re not really treated any differently than a traditional deployment.

Ok so… AFAIK it’s not possible to share storage pools between separate Commvault deployments.  However, what if I could configure the Commserve instances in LiveSync which would allow central control over the combined Media Agent / VSA components of the entire system?  it would fit the bill perfectly.  Now is that possible? 

You can have multiple HyperScale appliances within a single CommCell environment. For your Site A Storage Policies, you configure a Secondary Copy to your Site B Storage Pool, and vice versa. Just like any other CommCell environment. 

What region are you in?

Thanks for reply but I’m missing something! From the following link Multi-CommCell Environments

A CommCell environment is the logical grouping of all software components that protect, move, store, and manage data and information. A CommCell environment contains one CommServe host, one or more MediaAgents, and one or more clients.

Given a Hyperscale X appliance contains a CommServe and Media Agents it could be considered a Commcell environment. It is possible to manage multiple Commcell environments using Universal and Global Command centre with a Primary and other Service Commcells but I don’t know if it's possible to share resources between them. From previous experience no, but in this case would like to be proven wrong I’m in Asia Pac and think we will explore this with sales. 

Or were you possibly talking about GRC??

The HyperScale appliances can contain the CommServe - but they don’t have, you can connect an appliance to an existing CommCell. 

For your use case:

Site A HyperScale could be deployed with the CommServe on the appliance. 

Site B HyperScale could be deployed, and joined to the Site A CommServe. 

Site B HyperScale could then host the Standby CommServe (LiveSync). 

Based on your location, I recommend you reach to Fazeel. He is really knowledgeable. 

​@Faz Maybe?

Righto! Now I’m seeing the picture! So, it is possible to run two hyperscale appliances in LiveSync which would certainly fit our requirements! Thanks for the efforts! 

Do you have a third, possibly a Corp HQ, datacenter with decent bandwidth and access to both of these remote sites?
Even better, is there an existing (brownfield) infrastructure in an existing HQ DC that you could leverage?
Unless there is a justifiable business driver to put the Commserve “local”, there is no technical requirement to have the Commserve hosted onsite at either of your “new” sites. 
Also, if you don’t need high resiliency at either site, I’d be tempted to consider a conventional (non-Hyperscale) device at each site. A COTS Intel based 2U server can hold several hundred TB of internal disk.
If you prefer, you can preinstall with Windows MA config and cross-replicate between the two (using a 3rd site Commcell)
Installation and management complexity is reduced versus Hyperscale, if you’re not on site, and are remotely directing local hands/eyes.
FREL is required for file level recovery if using Windows MAs. 
One benefit of Hyperscale (or conventional MA running Linux) is that these MA nodes intrinsically provide the service otherwise delivered by FRELs, eliminating the extra resource to manage.