You are pretty much on the right track.
You could do it like this:
Create the triangle shape ( Or in your case, delete the second triangle, you only need one at this point ).
Path selection tool and click the triangle once and press Shift + Alt + left arrow key.
- Left arrow key: Moves the path to the left
- Shift: Makes sure that the 1 Left arrow key click equals 10px to the left.
- Alt: Makes sure that when the left arrow key is clicked, the currently selected shape is copied as well
( after that you may want to press just the the arrow keys few more
times to position it perfectly... )
From the tool options you will find icon for path operations. Select:
Subtract front shape( In older versions these icons will all be in the tool options sitting side by side, because that's how they roll... )
...that will subtract from the previous path.
This all happens inside one shape layer:
...and the same method works in older versions as well, as long as you start with a shape layer.