Mitred knitting, mitered knitting, domino knitting, modular knitting - all variations on a theme.
Here are the procedures I followed to join my second square. These instructions will show you how to join a second basic mitred garter stitch square in two colours to the first. (See Basic Mitred Square to see how to create the first square.)
I continued to use sport weight cotton yarn on 2.50 mm needles, and the next diamond also turned out to be about 4 inches (10 cm) across the diagonal (from point to opposite point).
In the charts below, A1 is the first mitred square (diamond), A2 the second, and so on.
If you continue working the instructions above, you will end up with a strip of diamonds joined together, starting from the lower right and ending at the upper left, when viewed from the right side (see below). Oriented this way, the stripes of different colours will look like Vs, and the squares will look like diamonds (pointed at the top, bottom and sides).
A4 A3 A2 A1
If you turn your strip onto its long side, you'll have a long thin rectangle of squares (flat edges on top, bottom and sides). The stripes will look like backwards L's on each square.
A4 A3 A2 A1If you turn your strip onto it's short side, you'll again have a long thin rectangle of squares, but the stripes will look like proper L's.
A4 A3 A2 A1
Obviously, I'm not covering all possible layouts of the strip, but suffice it to say that once you've made 3 or 4 mitred squares in a row, you should look at your work from different perspectives to see which pleases you most, and / or which suits your project best.