Why do I need to use two pipes, one inside another?

You don't. It's just that most PVC pipes with suitable sizes for rockets are for electrical wiring and thus are not designed to withstand a great deal of pressure. These pipes also tend to be low in quality and have a tendency to fissure. Adding the second pipe inside the first serves to help with distributing the force of the pressure. I have been using 25 mm electrical tubing for my rocket motors for years and I can tell you that if it doesn't crack during the powder packing, then it won't burst in flight either, as the heat generated softens the plastic and makes it more malleable. The motor will slightly expand from the heat and pressure, but it should not burst.