Headers and crossover pipes (X-pipes and H-pipes) are attempting to do the same thing but use different methods. They both attempt to reduce back-pressure to help the engine breathe on the exhaust stroke.
Headers are reducing back-pressure by providing a longer pipe for each cylinder so that the exhaust gases of the other cylinder on their previous exhaust-stroke aren't choking the cylinder currently on it's exhaust-stroke.
Cross-over pipes are reducing back-pressure by allowing one exhaust bank to use the other bank's exhaust pipe. The best effect is where the pressure is the highest, and that is closer to the engine, definitely before the muffler, as exhaust pressure slightly decreases along the length of the exhaust pipe.
To answer your question, well in theory the best place would be before the resonator, catalytic converter and muffler, but after the header. Practically, you need to see where you have the space.
Also, there would be no need to have more than 1 crossover pipe, since the operation of the crossover pipe will more or less equalize the pressures between the 2 pipes. If the crossover tube is of sufficient diameter to let enough exhaust bleed into the other exhaust pipe, nothing will be gained by a second crossover pipe.
Lastly, you asked if the crossover pipe should ever be placed after the exhaust. Maybe you meant 'muffler', because everything from the header to the exhaust pipe tip is the exhaust system. 'After the exhaust' is the air outside. :)
