First questions first - do you want to be DVM who does research in toxicology, or do you want to be a veterinary toxicologic pathologist? Or a veterinary pharmacologist with a focus in toxicology? Do you want to be "boots on the ground" doing actual experiments or do you want to be the person behind the microscope looking at tissues and writing reports? Where are you currently in your training? Do you already have a PhD?
The DVM/PhD is the combination if you want to be primarily driving toxicology research. This type of job would most likely be being a primary investigator in a lab - research, writing grants, etc.
If you want to be a DACVP (board-certified pathologist) or DACVCP (board-certified veterinary pharmacologist) doing tox path you much better off doing the DVM + residency +/- PhD (the latter being debatable based on whether you want to be more on the service/consultant side versus the research side, as long as you stay in industry).
Take a look at the job postings on each specialties national website to get an idea of what types of positions are offered, and what tox path jobs are available specifically. Here is an example of a tox path focused job and what it would entail:
http://www.acvp.org/index.php/en/co...veterinary-toxicologic-pathologist&Itemid=101
Like lily said, having a PhD already can definitely make it more difficult to get a pathology residency as so many of them (mostly anatomic) are combined programs now. Although there are exceptions - a few programs are not combined, and a few are flexible - one of my resident-mates already had a PhD and our was a combined program although he was an exception.
If you are committed to post-DVM specialization, I would personally recommend pursuing the PhD after vet school or in combination with a residency - my personal opinion of DVM/PhD programs is not very positive (although I encourage you to get more opinions than just mine).
