I tried to fit 2 single din units (1 dvd player & the other a motorized screen). I was told that this wouldn't be possible due the bottom half of the back having the heating pipes there. If you look at the back of the original bmw system you will see that the top comes out more than the bottom half. The other way to do it is have 2 single units & have the dvd player as a single din unit where the bmw unit it & have the screen where the air vents are just above it. Seen this done and looks fantastic & I will get this done in my e39 too.
Hope this helps.