Most teams DON'T run a combo defense BECAUSE it essentially takes double the practice time.
That's the key disadvantage of it.
The advantage is that it does offer you options - you can run both at once or either defense in any given game. However, the practice time you spend on the second defense is probably more detrimental to your player growth by losing individual practice time than it is worth - i.e. you won't win more games by using two defenses, but you might if your players improve faster.
Again, that's why most people don't run combo defenses.