How old is your bmw, because mine has the connection port under the central arm rest.
Well mine's got 2 connection ports so there

and the use of 2 ports for one unit is?

Erm......
In case you have an extra unit maybe? It's just got 2 aux inputs that's all. Big waste of time really. There's 2 cigarette lighter charger thingys as well. Strange.
It's a shame they're behind the centre arm rest rather than in it like your 3 series. Much better where your's are so you can leave your iPod in there all the time.