So I'm looking at the Magnaflow exhaust system for my 550, like listed here:
http://www.modbargains.com/MagnaFlow...t-BMW-550i.htm

So it has dual mufflers, but the rear ground effects have only one opening for the exhaust tips. So how does that work?

And no one online seems to have a picture of it installed.

Depending on your current bumper you could get a different diffuser, an m5 bumper or an m5 replica bumper. All of these options will allow exhaust tips to poke out on each side. You can also cut your current bumper. Up to you.

did you go with the magnaflow i was thinking about running it too i had magnaflow on my trailblazer ss and it sounded great.

No I went with RPI, ordered but haven't installed yet. I couldn't get enough info on the Magnaflow system, no pics, anything. I even called Magnaflow and they said that part was still 'upcoming' and were waiting to see if there was enough interest to produce it??