When I knit a shawl in two-color brioche, starting from the neck, I first make a tab in two-color brioche rib. Then I pick up stitches from the edges of the tab. Here is how I picked up the stitches for Humus shawl. I don’t know if there’s an official name for the method. I’m calling it brioche rib tab, since the tab is knitted in brioche rib, and the cast-on method resembles the garter tab cast-on.
My method of casting on using two colors and the cable cast-on method is a bit different from what seems the standard two-color cable cast-on. There is a video on YouTube on how I do it.
For short, I make the slipknot in only one color – it is the color I knit first on later rows – and I pick up the yarn for the next stitch from under the other color. The latter makes the yarns twist around each other, so if you intend on casting on more than a dozen stitches, it’s more convenient to pick up one of the colors from over the other color.