You might look into something from a 450SL or Euro 500SL. www.timevalve.com has illustrations. Those systems look like dual exhaust except at the cat and rear muffler. I don't think there's room under a 107 for left and right exhaust pipes.

Oops, you're outside the US. The only thing left is to split the rear muffler into two separate mufflers. Then you'll have full dual exhaust.

Sixto
91 300SE
86 300SDL (due soon)
81 300SD (for sale cheap)