Compute, it just depends....sometimes I will train chest and triceps on the same day when I really want to exhaust both groups of muscles--I make sure that I use exercises that will work both groups.
Other times, I want to make sure I get a good workout in just one of those areas. For example, I may want to really work on my chest because it is lagging behind, so I just do primary chest exercises and do my tricep training on a bicep day.
Also, keep in mind, that the shoulders are also involved with a lot of chest and tricep exercises, so if you also train shoulders on the same day, exercises like bench press are really going to suffer....