We set that up a few years ago but have since moved away from it because ArcGIS Server machines would have some fits getting lost from the shared content folder and extra overhead of maintaining all the pieces.
We just eliminated the duplicate machines and doubled the ram/cpu resources in the remaining virtual machines.
The active-active architecture, (also known as a silo'ed architecture) described in the link you posted can't be federated with Portal for ArcGIS. You may or may not be interested in federation, but it's an important point for anyone considering that type of deployment.
I'm beginning to think this is such a hot idea. I'm not interested in failover per-se, I'm interested in upgrad-at-my leisure: Take one server down, upgrade it, take the other down, upgrade.....but that's not going to work with the data store on a drive share.
Active/active can definitely help with rolling upgrades, as each machine participates in its own site and you can remove/add them as needed. What do you mean when you mention the data store is on a shared drive? Only the arcgisoutput directory is shared between the sites, and it's fine if one site is upgraded and the other isn't yet.
Where is the documentation that shows what Deployment scenarios—ArcGIS Server (Windows) Installation Guide | ArcGIS Enterprise are not able to be federated?
Would Multiple-machine deployment with ArcGIS Web Adaptor—ArcGIS Server (Windows) Installation Guide | Arc... allow for bringing one machine in the site down for an upgrade?
No, a multiple machine site won't support rolling upgrades. In order to maintain availability during an upgrade, you can follow the majority of the steps outlined in the blog about migrating machines using the DR tool:
After step 8, you can upgrade the standby environment to the latest version and when ready, redirect traffic to it. Unfortunately, this does require more hardware and machines to implement, but it's required due to the stateful nature of the software.
One potential way for the multi-machine "upgrade at your leisure" requirement, is to unregister each machine and upgrade it at your leisure, and keep doing this(machine after machine) until you reach the last machine. Only when you upgrade that last machine would the site configurations really get upgraded. Post last machine upgrade, you can add all the other machines back.
This is under the assumption that you can afford some period where you have less number of machines(it can be staggered based on an interval that you prefer). Also, you will need to re-import any certificates that were used by each of the machines. This is not ideal, but just a thought since you mentioned upgrading each machine at your leisure.