Here's an easy, no calculation method courtesy of Matt Kloskowski. This takes much longer to describe than to do:
1. To make things easy, set your foreground and background colors to the default (black/white).
2. Create a new blank (white background) document of the appropriate size.
3. Create a new, transparent layer on top of the background layer.
4. Select the Marquee Tool.
5. Draw a box on left side for the first photo of whatever size and shape you want.
6. Press Alt-Backspace to fill the shape with black.
7. Use the Marquee tool to draw a box for the second photo (also on the same transparent layer) and fill with black.
8. Open the photo for the left side and drag it on top of the layer with the black boxes.
9. Press Ctrl-T to get the Free Transform handles and resize the photo so that it is slightly larger than the left side black box. Don't worry if it spills over to the right side, that will be fixed in a moment. Only use the corner handles so you don't distort the image!
10. Press Ctrl-G to Group the top and middle layers. The black box(es) will act as a layer mask for your first picture.
11. Open the second (right side) image, drag on top of everything else, position over the second black box, resize as needed, press Ctrl-G to group and mask.
12. If one image overlaps into the other box, reorder the top two layers as needed to hide the overlap.
13. Use the move tool on each of the images to get the precise positioning that you require.
14. If you now want a black background rather than while, change the background layer to black (or any color desired).
Edit -- Additional: for precise sizing and positioning of your mask boxes, turn on the rulers (Ctrl-Shift-R), then click on the ruler and drag onto the image to create guide lines where you want the margins and the space between the boxes.