5 hours might seem pretty long but if you consider that you won't need to go to/from the airport as the train stations are in the cities, taking a flight won't take much less time.

Moreover 35€ is probably what it will cost you to go from Paris to CDG and from DUS to the city itself (so without the plane ticket itself).

for 2 people, I think that 2 seats next to each other are much better : you won't have to share the leg room. And in 2nd class, the seats that are face to face go by 4, so you might have some people next to you.