This example specifically relates to my "St Abbs Brioche Beanie" (the pattern of which is available through my workshops and is soon to be released on Ravelry), but the general principles can be applied to other projects. As you can see from the photos below, the beanie has been knitted in two colours, both of which were dyed by Queen of Purls: "Flowers of Sulphur", which I will lazily be referring to as "gold", and "Space Junk", which I will call "grey".
Thus we work back and forth, from the right to left hand side of the seam, creating the rungs of ladder stitch in the usual way. However, it is essential to check BOTH sides of the fabric at regular intervals: On the predominantly gold side, the seam should be formed by the joining together of grey purl bumps; remember not to over-tighten the stitching (or make the rungs of the ladder too narrow) so as to prevent the fabric from being distorted or the columns of gold "v's" been pulled too close together (which will make a more obvious break in the pattern). On the reverse of the fabric, we are aiming to create an even column of (ideally single) grey "v" stitches. (A seam that is one "v" wide more readily matches the columns of single "v's" that already exist in the fabric.) As previously said, it may be necessary to alter the exact positioning (i.e. the distance from the edges) of the sewing together to ensure consistency of the seams on both sides of the fabric.