There may be a simpler way, but this is how I do it. If both pictures are the same size...
Start with the one you want on the left. Click on the image menu, then canvas size. Change the width to 2x what it is. At the bottom of that box, you'll see a grid. Click the top left box (actually, any of the left boxes if you're only doing side-by-side). That anchors the image to the top left part of the enlarged canvas. Click ok. You'll have a white space to the right of the first picture.
Open the picture you want on the left. Go to the select menu and click all, or hit ctrl+a. Then copy, ctrl+c.
Go back to the first picture, and paste, ctrl+v. Select the move tool, and drag the pasted picture into position.