Executing two callbacks of two subscribers parallelly. How?

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If one sub1.callback is on execution and meanwhile msg arrives on other sub2. I want to process sub2.callback parallely. Is it possible? How?

ros::Subscriber image_sub= nh.subscribe("/rgb, 2, &class::imCallback, &class);
ros::Subscriber depth_sub= nh.subscribe("/depth", 2, &class::depthCallback, &class);

I read many posts on callback queues, MultiThreadedSpinner, boost. I am confused which way is good for my application !!