How to program two NXT motors to run at the same time?
In NXT-G if you place a block after another block it will run in succession. To get two motors running at the same time you can place two motor blocks, one at the start and the other just above or below. Drag the sequence beam (the white Technic beam) to the second block to complete the circuit. 

Don't forget to make sure the motor blocks are controlling separate ports and that both ports on the NXT brick are attached to motors. 

You could just use one block to control the two ports but they would both have to be running in the same direction.