its one of those things where the lingo gets confusing...
dump pipes are really the first part of the exhaust that the turbo bolts onto. this part is where there are usually 2 pipes that join a bit further on in the pipe. one is smaller, for the wastegate, and one is larger for the turbine exhaust gasses.
here's a pic for a r32 gtr:
the dump pipes or 'extension' pipes bolt on to the 'front' pipe.
front pipe comes after the dump, and before the cat.
heres a pic, also for r32 gtr:
the pic CRAVED posted is a dump and front pipe all in one.
it shouldn't matter if its 2 or 1 piece and will differ from car to car, make to make.
hope that helps
dan