A lot depends on the pattern you want. Here are a few ideas:
1) Pour one color and fill half a mold. After that sets, pour the other color on top. No risk of glue failure and excellent seam.
2) Take the first color and turn it on the lathe. Part or carve away sections for the other color and recast.
3) Cast one color and cut up some solid pieces. Cast the other color and before it sets, imbed the pieces of the first color.