PLEASE HELP: 2 cameras subscriber & Depth perception code problem

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I think what you are searching for is the message_filters package espacially the Time Synchronizer. Try this example:

#include <message_filters/subscriber.h>
#include <message_filters/time_synchronizer.h>
#include <sensor_msgs/Image.h>

using namespace sensor_msgs;
using namespace message_filters;

void callback(const ImageConstPtr& image1, const ImageConstPtr& image2)
{
  // Enter the cv_bridge here like you did before
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "vision_node");

  ros::NodeHandle nh;

  message_filters::Subscriber<Image> image1_sub(nh, "image1_topic", 1);
  message_filters::Subscriber<Image> image2_sub(nh, "image2_topic", 1);
  TimeSynchronizer<Image, Image> sync(image1_sub, image2_sub, 10);
  sync.registerCallback(boost::bind(&callback, _1, _2));

  ros::spin();

  return 0;
}