does one HAVE to declare both moveactions at the start of the turn?
No, you don't HAVE to declare both of your actions up front. You can complete the first action before deciding what to do with the second.
In the case of move actions specifically, you CAN declare both at once (ie: declare one Move, then immediately "interrupt" it to declare a second Move), gain twice your MP and then use them all as you see fit, OR you can declare them one after the other. The choice is there for the acting player to decide.
The down-side of declaring both at once is that if you get immobilized or something (ie: Web Trap) and your Move action is cut short, you will lose BOTH move actions. FFG has clarified this, in the FAQ I believe. The down-side of declaring one after the other is that you might get caught by a Pit Trap in between moves if you don't keep an extra MP lying around. Of course, the hero could also "interrupt" his first move right before spending the last MP in order to declare his second Move, if that's what he wants to do. Then he'll have the MPs he needs before you Pit Trap him.
All that said, the specific case you presented in the OP is illegal because he can't declare his second Move in reponse to Pit Trap, as Dam said. The Pit Trap card needs to be resolved first, before anything else happens.
The hero also couldn't spend a fatigue point for an MP in response to Pit Trap, btw. But he COULD spend a fatigue earlier in the turn so he has an extra MP lying around in case you try to Pit Trap him later.