What modifications do I need to fit a Right + Left exhaust system on a car with a single side exit stock system?
The correct way of operating depends on the car model.
If there’s enough space under the frame for a second muffler or for a secondary pipe the modification or replacement of the bumper is enough.
In the exhaust system description is specified if the bumper or diffuser must be trimmed, a job for a competent installer, or is necessary to buy the original spare bumper from a different model of the same car (for example the Audi S5 bumper on the A5).
If the necessary space is occupied by some components, like covers or blanks, these must to be removed or modified
Eventual mounting brackets are provided directly with the exhaust.
Here is a sample of a quad pipe exhaust for a car that has a single side OEM exit (BMW E90 320i):