gravel on one side, and sand on the other ????

I have seen pictures of it before. People put little sand "pits" in their tank for some bottom dwellers so I dont see how 1/2 and 1/2 would be a problem.

Many pro aquascapers use plastic dividers between substrates to get a great look. However, from what I understand, even with the dividers they will mix eventually.

sand has the nature of flowing though cracks and crevices, and will eventually find its way into the gravel.

I would imagine with water changes the sand would get stirred up and get mixed up in the gravel.

Over time even if you are careful, chances are the gravel and the sand will become too mixed for your liking. Then you may want to change out the whole thing which is trust me a real pain. I would choose one or the other and go from there.

Marinemom

sand. looks better imo

With some planning, I'm sure it can be done... if you had some sheet acrylic to use and some silicone glue (not sure about whether the aquarium sealant most lfs's sell will adhere correctly to acrylic... I'd check that out before trying) I'm willing to bet you could form a divider in the bottom of the tank, but you'd have to have the tank both empty and dry to do so. (probably take a couple days to cure too) If you made say a 1.5" high barrier, and filled each side with an inch or so, you'd probably avert most of the mixing you'd encounter without gluing/having a barrier, but invariably, some sand will find its way into the gravel, be it because of the fish physically moving it, currents, water changes, etc. Its just bound to happen... how prepared you are for it before you set up a tank with both may determine how long you will have before you have a decent bit of mixing. I myself made the switch from gravel to sand something like 5 months ago, and I've loved the look of the sand ever since.