A little over a year ago, I decided to do something a little creative with my substrate. I have always used sand or gravel substrates in all of my tanks but I've never had a tank with a combination.
As you can see in the photo at top, there is a large anubias (the plant in the right rear corner), which I've had for years. I didn't want to part with it, so I needed to leave a healthy amount of gravel on the right end of the tank where it's planted.
Eventually, I decided to go with all sand, except for where the anubias is. Thus that little patch of gravel on the right was the only gravel left.
The tank has changed immensely since I added all of the white sand. Not only have I completely redecorated the tank (as you can see in the photo above), but it also has a lot of new cichlids. The anubias, by the way, is a bit smaller because I trimmed it and planted offshoots beside it.
Needless to say, the clear delineation between gravel and sand didn't last long, though I wouldn't have expected it to if I had numerous substrate sifters and stirrers. However, I have a few rambunctious cichlids that surprisingly stir the sand up significantly. The result is that the whole right end of the tank is now pretty much nothing but a mix of sand and gravel. The photo above was taken in November 2016, and you can still make out quite a bit of gravel. Now it's almost all mixed up.
The point is, if you're wanting to keep multiple substrate types segregated, you'll probably need to set some clear barriers between them (hint: think of a solid fence) in order to make it difficult for your cichlids to mix them. Mixing not a bad thing, but it may not be as aesthetically pleasing as you had planned. If you keep Geophagus species (eartheaters), a clear barrier of some kind won't be enough.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub