Our firm works with the forensic accountants firms as partners on projects because they really are two different skill sets and practice areas. The more skills you have in one area is IMO better than many skills spread out in many areas.
The question of whether or not forensic accounting or fraud examination skills and knowledge is useful for a digital forensic examiner depends on what type of cases you primarilly are going to be working on.
If you are in a fraud/white-collar crime environment, then it will certainly help you in understanding what you find when conducting a forensic examination, and may even enhance what you provide to investigating officers.
However, I am only talking about a fundamental skill and knowledge level, and am not saying that it would be practical to be both a forensic accountant and a digital forensic examiner, as both are specialised disciplines in their own right.