I was just reading a book today that was talking about this, and they suggested twisting the two yarns together at the end of the row. For example, you're knitting with the wheat color and then you switch to the charcoal. At the end of each charcoal row you twist the tail of the wheat with the tail of the charcoal & then drop the wheat & start knitting with the charcoal again. That way, the wheat is carried up the side and incorporated into the work as well. Then, when you're ready to switch back to the wheat, you simply switch and start knitting with it.
Now, I've never actually tried this myself, so I'm probably not the ideal person to be giving you advice, but I thought it seemed like a good concept.