I believe the Crop tool works like it does in desktop Photoshop: You can actually expand the canvas rather than just crop.
This would probably work best if you know the size of your image (in pixels, particularly the width) so you can fit that sucker as soon as you expand the canvas.
Add the first photo as you would in any PS Touch project. Tap the ampersand icon ("&") in the upper right (2nd icon from the right). Tap, "Crop." Tap the rightmost center handle and drag it to the right. Note PS Touch will let you know the new size in width and height. (Note also that you have a max width and height of about 4096 x 4096 pixels; plan accordingly.) Tap the checkmark.
Tap the "add image" icon in the upper left (2nd icon from left). Position the new image to the right of the original image. (It should also "snap" by edges to make it easier.) Tap the checkmark. You should be done at this point. Layer, add effects, etc.
In addition to Warunicorn's post, I have shown below how to place photos side by side.
1. Make a new project. Select New Project, then make the new dimensions for your new project.
2. To add a new layer, press the "+", then select "Photo Layer".
3. Select the location of the photo, then select "Add".
4. With the new photo placed, you can transform the photo. Here I just stretched the photo, but if you are worried about distorting the photo, you may want to crop the photo as warunicorn suggested.
5. I placed the photo to the left side when I was done transforming it. Press the "check" when you are done.
6. To add another photo, you need to press the "+" to add another layer, and "photo layer".
7. Select the source for the new photo.
8. Now you can transform the photo again.
9. Below I have moved the photo to the right. Press the "check" when you are done.
10. Below is the finished product!
Please post back if you have any further questions,