OK, but from a visual perspective, the gravel and sand will in time mix together, unless there is some sort of divider siliconed to the tank base to separate the substances. I really would suggest all sand with wood and river rock as accents.
And earlier you asked about plant fertilizer...yes, in a new aquarium there is obviously no (or very little) organic matter, so the plants need a good start. Substrate rooted plants like swords will benefit from substrate tabs; the best I have found is Flourish Tabs. And Flourish Comprehensive Supplement for the Planted Aquarium is an excellent liquid plant supplement. I always use this initially at two doses for the first week, then once there are fish I reduce the liquid.