Altered code to properly handle asymmetric twin swing dice

Fixes #1467.

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/555/
Yeah, i meant "nit" in the sense of "i'm not going to reject the pull if you don't want to do it".  But i do think it would be a good idea, so if you're willing to do it, i think it'd be an improvement.  I feel like a lot of the complaints about our API have been about lack of internal consistency, so we'll want to make this change eventually if not now.