You must put Nimbus on top of DD Miami City in the scenery.ini for that both products are visible.
(sorry for the correction Stephen).
This way DD's rather simple airport will disappear below by the much better Nimbus airport,
but DD's other 2 regional airports will still show.
From this point you can add or remove objects as mentioned by Stephen via the Overlay Editor Tool.
You don't want to change things with Nimbus as that one is perfect already, so make sure you open DD_Miami_City_XP's .dsf file.
The Autogen you may need to look over and revise, as some 1000 feet tall skyscrapers will eventually pop up along the Beach,
so use the exclusion zones function in the overlay editor to organize forests, residentials, roads and office facades.
it's a bit of a work to simulate the real thing, if that is what you want. I did want it caus i love flying MIA.
I have reworked most of DD's Miami for my own purpose, added Night Bridges over the bay, created the Stadiums (e.g. Merlin Park) to make this area timeless, or lets say independant from the need of future updates. DD's Miami off the box is a sloppy conversion from their previous FSX release and all the 3d-buildings were stolen from Google, that's why the Drzwiecky thing looks so terribly outdated and bad coloured when it comes to the textures. I talked to the guy from the polish developer , and it does not look like they have big interest in releasing updates on Miami.
To get a good Miami Feeling, the buildings must be repainted to white and nice and the beach areas require a lot of corrections. I ran all the building textures thru a photoshop emulator to whiten and contrasting them to a more realistic look. I also created the Miami Seaplane Base which was forgotten and which is being revived in real life with a complete overhaul and with new superdesign terminals.
I highly recommend using Level19 Bing Ortho4XP for the entire Miami area (Palm Beach down to Key Biscane) or even the whole of Florida down to key West,
the flight experience is breathtaking, if the geo-visuals are done nicely. DD's package does not show the beautiful sandbanks in the bay or in the outer beach sections.
But when using ortho over DD's own terrain you need to do a little delete-job on their own terrein tiles and that can cause errors if not done right.
Let me know when you reach this point.
Here a few pics of how the result should look like (x-plane 11.30+), some water areas work in progress and number of objects slider on medium, autogen works in progress: