What Paul said. Of course, with 2 workstations, you could have 2 artists working concurrently. I for one believe that you should put as much hardware resources to work as you possibly can for each artist as a force multiplier. so if you're talking about resources for a single artist I'd definitely say go with 2 Titans/1 box.
Of course, remember that a good up-to-date CPU would be more than the cost of that additional Titan!
