No you cannot and should not ever use Frontline and Seresto at the same time.
If you use Frontline and Seresto at the same time on your dog then you could overdose the dog with the chemicals contained in the Frontline and Seresto.
Both Frontline and Seresto contain strong chemicals to kill off fleas and by using both of them on your dog at the same time increases the risk of overdosing and potentially killing your dog.
Only use either Frontline or Seresto at one time.
Never use both Frontline and Seresto at the same time because your dog could get deathly ill or even die.