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The following code works for me. two usb cameras, opencv2.3.1, MSVS2008, windows7 32 bit

#include "highgui.h"
#include "stdio.h"
int main()
{
   CvCapture * capture1=cvCaptureFromCAM(0);
   CvCapture * capture2=cvCaptureFromCAM(1);
   if(!capture1)
   {
      printf("Cannot find device1!\n");
      exit(0);
   }
   if(!capture2)
   {
      printf("Cannot find device2!\n");
      exit(0);
   }
   IplImage * image1=0;
   IplImage * image2=0;
   cvNamedWindow("Grab1");
   cvNamedWindow("Grab2");
   while(1)
   {
      image1=cvQueryFrame(capture1);
      image2=cvQueryFrame(capture2);
      if(!image1||!image2)
         break;
      cvShowImage("Grab1",image1);
      cvShowImage("Grab2",image2);
      int key=cvWaitKey(10);
      if(27==key)
         break;
   }
   cvReleaseCapture(&capture1);
   cvReleaseCapture(&capture2);
   cvDestroyWindow("Grab1");
   cvDestroyWindow("Grab2");

   return 0;
}

The following code works for me. two usb cameras, opencv2.3.1, opencv2.1, MSVS2008, windows7 32 bit

#include "highgui.h"
#include "stdio.h"
int main()
{
   CvCapture * capture1=cvCaptureFromCAM(0);
   CvCapture * capture2=cvCaptureFromCAM(1);
   if(!capture1)
   {
      printf("Cannot find device1!\n");
      exit(0);
   }
   if(!capture2)
   {
      printf("Cannot find device2!\n");
      exit(0);
   }
   IplImage * image1=0;
   IplImage * image2=0;
   cvNamedWindow("Grab1");
   cvNamedWindow("Grab2");
   while(1)
   {
      image1=cvQueryFrame(capture1);
      image2=cvQueryFrame(capture2);
      if(!image1||!image2)
         break;
      cvShowImage("Grab1",image1);
      cvShowImage("Grab2",image2);
      int key=cvWaitKey(10);
      if(27==key)
         break;
   }
   cvReleaseCapture(&capture1);
   cvReleaseCapture(&capture2);
   cvDestroyWindow("Grab1");
   cvDestroyWindow("Grab2");

   return 0;
}

The following code works for me. two usb cameras, opencv2.1, MSVS2008, windows7 32 bit

#include "highgui.h"
#include "stdio.h"
int main()
{
   CvCapture * capture1=cvCaptureFromCAM(0);
   CvCapture * capture2=cvCaptureFromCAM(1);
   if(!capture1)
   {
      printf("Cannot find device1!\n");
      exit(0);
   }
   if(!capture2)
   {
      printf("Cannot find device2!\n");
      exit(0);
   }
   IplImage * image1=0;
   IplImage * image2=0;
   cvNamedWindow("Grab1");
   cvNamedWindow("Grab2");
   while(1)
   {
      image1=cvQueryFrame(capture1);
      image2=cvQueryFrame(capture2);
      if(!image1||!image2)
         break;
      cvShowImage("Grab1",image1);
      cvShowImage("Grab2",image2);
      int key=cvWaitKey(10);
      if(27==key)
         break;
   }
   cvReleaseCapture(&capture1);
   cvReleaseCapture(&capture2);
   cvDestroyWindow("Grab1");
   cvDestroyWindow("Grab2");

   return 0;
}

    #include <iostream>
    #include <opencv2/highgui/highgui.hpp>

    int main()

    {
    cv::VideoCapture cap1, cap2;
    cv::Mat frame1, image1, frame2, image2;

    cap1.open(0);
    cap2.open(1);
    if( !cap1.isOpened() )
    {
        std::cout << "Could not initialize cap1"<<std::endl;
        return -1;
    }
    if( !cap2.isOpened() )
    {
        std::cout << "Could not initialize cap2"<<std::endl;
        return -1;
    }
    for(;;)
    {
        cap1 >> frame1;
        if( frame1.empty() )
            break;
        frame1.copyTo(image1);
        cap2 >> frame2;
        if( frame2.empty() )
            break;
        frame2.copyTo(image2);
        cv::imshow( "Img1", image1 );
        cv::imshow( "Img2", image2 );

        char c = (char)cv::waitKey(10);
        if( c == 27 )
            break;
    }

    return 1;
}