OpenCV displaying 2 images adjacently in the same window

EDIT: Here's how I'd do what you want to do:

Mat left(img_matches, Rect(0, 0, 1088, 2208)); // Copy constructor
imgorig.copyTo(left);
Mat right(img_matches, Rect(1088, 0, 1280, 2208)); // Copy constructor
imgorig2.copyTo(right);

The copy constructors create a copy of the Mat header that points to the ROI defined by each of the Rects.

Full code:

#include <cv.h>
#include <highgui.h>

using namespace cv;

int
main(int argc, char **argv)
{
    Mat im1 = imread(argv[1]);
    Mat im2 = imread(argv[2]);
    Size sz1 = im1.size();
    Size sz2 = im2.size();
    Mat im3(sz1.height, sz1.width+sz2.width, CV_8UC3);
    Mat left(im3, Rect(0, 0, sz1.width, sz1.height));
    im1.copyTo(left);
    Mat right(im3, Rect(sz1.width, 0, sz2.width, sz2.height));
    im2.copyTo(right);
    imshow("im3", im3);
    waitKey(0);
    return 0;
}

Compiles with:

g++ foo.cpp -o foo.out `pkg-config --cflags --libs opencv`

EDIT2:

Here's how it looks with adjustROI:

#include <cv.h>
#include <highgui.h>

using namespace cv;

int
main(int argc, char **argv)
{
    Mat im1 = imread(argv[1]);
    Mat im2 = imread(argv[2]);
    Size sz1 = im1.size();
    Size sz2 = im2.size();
    Mat im3(sz1.height, sz1.width+sz2.width, CV_8UC3);
    // Move right boundary to the left.
    im3.adjustROI(0, 0, 0, -sz2.width);
    im1.copyTo(im3);
    // Move the left boundary to the right, right boundary to the right.
    im3.adjustROI(0, 0, -sz1.width, sz2.width);
    im2.copyTo(im3);
    // restore original ROI.
    im3.adjustROI(0, 0, sz1.width, 0);
    imshow("im3", im3);
    waitKey(0);
    return 0;
}

You have to keep track of what the current ROI is, and the syntax for moving the ROI around can be a little un-intuitive. The result is the same as the first block of code.

As the height (rows of Mat) of the images are same, function hconcat maybe used to horizontally concatenate two images (Mat) and thus can be used to display them side-by-side in the same window. OpenCV doc.
It works with both grayscale and color images. The number of color channels in the source matrices must be same.

Mat im1, im2; // source images im1 and im2

Mat newImage;
hconcat(im1, im2, newImage);  // <---- place image side by side

imshow("Display side by side", newImage);
waitKey(0);

For the sake of completeness, vconcat can be similarly used for vertical concatenation.