Ive done that before. Not with the exact same soil that you are using, but two different substrates layered for effect.
I would use sand as the top layer. Regular play sand works great and it will keep the underlying substrrate in place. The biggest issue with layered substrates is that at some point the mix. Either from replanting or the acossional accidental siphon going a bit too deep wjhile cleaning, you are bound to bring some of the bottom layer to the top. This isn't always a bad thing. Sometimes the mix doesn't look half bad.
Anyway, yes it can be done. How you do it is sort of your decision based on what you want. IMO I wold not use gravel on the top.
Hope that helps!