What Do Two Symbols in a Square Indicate on a Cross Stitch Pattern?

If the pattern key does not not give specific directions about the color of the back stitch (or the half stitch in other cases), then you must use your judgement. Generally, the dominant color is the best choice. In this example, The ball is the dominant image. The back stitch should be done in the color indicated by the + symbol. In this case, that color is green.