As long as the backpack fits underneath the seat in front of you and the duffel bag does not exceed the carry on size then you are fine. Check the size and weight requirements for the airline that you are flying, as these vary a good bit. (Particularly the weight limit, not all airlines have one for carry-ons.)
What size backpack is a personal item on airlines?
Personal Item Size LimitsAmerican Airlines advises customers to travel with personal items that are no larger than 18 inches x 14 inches x 8 inches. If you are flying on Delta, be sure your personal item is 17 inches x 13 inches x 9 inches or smaller.