Why does my bike crank creak and come loose?
Square taper cranks are easily damaged if they a ridden loose. You may find that the only fix is to replace the crank. If after tightening to the correct torque they continue to loosen, they must be replaced. Over tightening, while tempting, is not the correct solution and will lead to maintenace problems down the track. (Essentially someone will have to remove the crank for some reason, it will keep coming loose after that and eventually be replaced. The process of fixing will take several attempts as the presumption will be the crank is in good condition when it is not) 

Take the bike back to the the shop. As the crank has come loose again, ask them to replace it. If they refuse, be prepared to take your bike back every two weeks until they install new ones. Do not "fix" the bike yourself. The LBS might claim it was something you did that caused the problem.