This mod has been done several times by other E90 guys. This is how I did it. My stock E90 M3 console came with an ashtray and no power outlets. Before starting this mod, move your seats all the way forward and sit in the back seat.
To remove the rear console, you need an upholstery tool that can depress male tabs that are part of the rear console part. If you don't have an upholstery tool, I suspect a credit card woould work too. There are 2 of these on the top of the console. To access them, open the armrest and press down in the 2 locations shown. The rear console will separate from the rest of the main console.
After the top 2 tabs are unlocked, you need to unlock the 2 lower clips. These are a slightly different design. Reach in from below and you can feel them. Unclip each of them (much easier than it sounds) and the rear console will be free. Unclip the wire going to the rear seat vents.
Remove the rear seat vents from your factory rear console by unlocking the clips from the inside out. Transfer the assembly over to your new twin-power rear console. Rather than re-use the ashtray, I purchased the rear seat oddments shelf and rubber shelf liner shown below.
To connect my new power outlets to car power, rather than use Scotch locks or hack apart factory wiring, I re-built the power harness assembly inside the console. I cut off the plug that came with the new rear twin-powered console and stripped the wire ends. I unplugged and removed the power connection on the left side of the rear console. You need to remove the 2 male pins in this white connector. To do so, you need a special BMW pin removal tool or equivalent. ! Before you do anything: Note the position and color of each wire.
Do not just mash the pin tool into the pins. That will not work. Instead, unlock the connector first. To do this, use 2 small flat blade screw drivers as shown:
The white plug assembly is actually 2 parts. If you look at the end where the wires enter the plug, you will see 2 pairs of locking tangs that slide over one another. You need to push the center of the plug assembly so that the tangs are on the other sides of one another. You may need a magnifying glass to see them. The above image shows 2 flatblade jewelery screwdrivers holding the plug in the unlocked condition. Once unlocked, you can easily remove the pins with the BMW pin removal tool.
I cut the factory male pins off and stripped the 2 ends of the wires, twisted all 3 ground (brown) and power (red) wires together, then crimped on new male pins. I also added heat shrink for a little more stress relief on the wires. When re-installing the male pins into the connector, make sure you get them in the correct position!
Once complete, reconnect and re-stow the power connection. Make sure you reconnect the factory wire to the air vents. The twin-powered rear console will snap back into place. Test your connections with the car in position 1/Acc.