The basic triangle is worked from the tip up to the neckline. Increasing two stitches every other row. See the top half of the diagram. I've marked an arrow for the direction of growth and labeled the sides.
In the bottom half of the diagram is the double triangle/ neck down triangle.
It is essentially two of the basic triangles worked side by side. Note the labeled sides and the arrows. They are just the same as in the basic triangle, except rotated. So, because you work the pattern twice, you have FOUR increases every other row. You can use a single stitch between the two triangles, or you can use the same number of stitches you use as a border if you were doing a single triangle. (if you do that, it counts as both the border of the first triangle and the border of the second triangle - you do an increase on either side.
ant to complicate it a bit more? put a rectangular panel between the two triangles and you have a "mock faroese" shawl.