Hi Yashika,
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?
Kind regards,
Jörg