Is you MDI factory fitted? If so, then it's wiring is part of the original loom with no additional plugs, so the new Bluetooth loom will simply plug in between the radio and original quadlock connector.
If they are both retro fitted, then both kits will come with a piggy back loom. I can't see any reason why you couldn't plug the Bluetooth loom into the radio, then the MDI loom into that, then the original harness into that, but it would be like a rats nest behind your dash. If that is what you are doing, I'd ask the suppliers it they could make you a combined piggy back loom to connect both devices.
