According to RealOEM, the 35i tip and the 28i Sport-line tip have different part numbers (must be sized a little differently). If you plan to retrofit I believe you must order two as they are not sold as a pair and must be welded (visit a muffler shop).
All 35i: black chrome - 18307559999
Sport-line 28i: black chrome - 18307635633
All others: brushed aluminum satin - 18307618683
Thread: Black chrome tips from the 135i to the 128i?