Seeking clarity regarding callback queues and ros::spin()

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Let's say that we have initialized 2 different nodes inside 2 different main functions. Also assume that each of these two nodes have one subscriber each, subscribing to the topic topic_a. We also have ros::Spin() at the end of each of these 2 main functions.

In ROS 1, there is a 1-to-1 correspondence between nodes and processes.

So only a single node can "live" in a specific process (or: program).

Programs do not share any of their internal state by default, which would include any (global) variables.

You also -- by default -- cannot have "two different main functions" in a single program. The compiler will not allow you.

In my answer below I've assumed that "two different main functions" therefore means: "two different programs".

Would the callbacks corresponding to each of these 2 subscribers be pushed to the same global queue and ros::spin() call from either of the 2 nodes would spin the oldest callback from this queue?

Due to the way operating systems isolate processes from each other, it's impossible for two nodes to share a single callback queue (or to share anything of their internal state really).

In the end, this is not really a "ROS question". Distilling it down to its essence, you ask whether process_A can access internal state of process_B. For any modern operating system, and without using special "tricks" (ie: shared memory), the answer will always be: no.