This is how it's done. PROPER.
Body Kit: M-tech with 335i diffuser and quad cut-outs.
Start off with a SuperSprint twin pipe muffler.
Cut the original tips off and reweld new pipes to it using one for the left side and one for the right side.
You make the correct cut outs for the magnaflow quad tips.
And then you weld them on.
Credits go to a fellow friend driving a 323i in Singapore.