This is basically a post-processing task:
Anyway, you'd need an image processing software, like photoshop (commercial) or like what I use, Paint.Net (freeware - with plugins to match Photoshop), which I'm using to help illustrate for you.
Paint.net if you want to get the software.
1. Determine the size of both your images (W X H, say in pixels, like on my Canon 500D, at the largest size, 4752 X 3168).
2. Create a new file, this time, make the size of the new image to be 2W X H. So, numerically its 9504 X 3168.
3. Turn off the Background layer in the Layers window (click the checkbox).
4. Layers -> Import from file (do it twice for both images)
5. Turn off the layer for the first imported image (in the Layers window).
6. Control + A, to select all elements in the 3rd layer, and move it to the right side of the black canvas so it will be in the position you wish it to be.
7. When done in step 6, click on the checkbox for the layer for the first image, and voila, both pictures are side by side.
8. Now, you need to delete the originally created Background layer, select it and hit the delete icon in the Layers window.
9. Lastly, as you have a 2-layer image in your picture, you need to flatten it to make it into a JPG or GIF file. Image -> Flatten
10. Save your file and give it the appropriate file name and JPG file name extension.
Hope this helps. (Unless you want to stitch a panaromic picture.)
Cheers!
