Unless you can put in dividers, the sand and gravel will get mixed up and eventually the gravel will rise to the top and the sand will be lost to view.
I toyed with the idea of using strips of perspex to keep a division between the two substrates. By coincidence, whilst considering this, I had an all-sand substrate with a flower pot with a plant in gravel in it. Clown loaches got in (loved it) and flicked all the gravel out onto the sand. This led me to reconsider the perspex divider idea in that any activity in the fish is still likely to mix the two with the usual result. I gave up on the idea and stuck with the all-sand substrate (but I still have the flower pot, I just put the gravel back in occasionally).
Martin