TweetLOL -Did this guy have big arms?
Theres many different ways to work you just really gotta find how your body responds. Many will want to train Chest and Tris together because yes most chest excercises work tris also. Having enough days for Recovery inbetween working a certain part is very important, not having enough time will cause overtraining and that can actually make you smaller the more you work out.
I cant think of any direct benefit or edge that working out both bis and tris in the same day would give anyone, what matters is really just preference, allowing recovery, and avoinding overtraining IMO