I need some help related to CQ5 Authoring. I have created 2 clusters in which each cluster is having 1 Author Instance and 2 Publisher Instances. The use-case here is to deploy author clustering between two author nodes part of 2 different clusters.
That should be fine. You have two authors and four publish nodes. Configure the master author to publish to the four publish nodes and then join the slave to the author cluster. If the author goes down, the slave will be automatically promoted.
When you run CQ 5.x (or AEM 5.6.1) in a TarPM cluster, you will always have "master - slave" relation between the cluster nodes. There is a single master and a number of slaves. That's on the lowlevel repository view. Of course you can work on all instances (it appears to a developer/admin, that all cluster nodes are equal), but deep down it's still a master-slave relation.
Second, if you want to run a TarPM cluster over multiple datacenters, you'll likely experience bad performance because of the latency. TarPM clusters rely on a stable and low-latency cluster connection (I always say: Latency less than 10 ms, better only 2 miliseconds; if you have higher latency, you'll get performance problems). With MongoDB the situation can be different, as then it's a MongoDB thing.
And: Having author and publish in a single cluster doesn't make sense, as Justin already pointed out. You have a single /content tree, but how would you split between the content, which is prepared by editors, and the content you deliver?
I am having two Author Instances on different datacenters. I am trying to work on devising a Disaster Recovery Strategy where I need a failover support in case any of the author instances stops working due to any datacenter related disaster.