While I don't use hemp bedding, my soils ARE sandy clays and clay-y sands.
I DO deep litter. Works great. With time, the sands and clays under the slowly composting deep litter will improve and actually become useful.
Yeah but that's outside in the run though, right? Not inside the coop? It wouldn't be the same benefit outside vs. inside. Outside you have soil to improve, so if your soil is sandy (or you've put sand in the run), over time the composting material will improve soil quality and make up for the sand.
Inside the coop though... that's a whole different story. Unless the coop has no floor and is sitting straight on top of the soil, but I don't think that's the case here. If it's an ordinary coop with a wooden floor, with sand on the bottom and deep litter on top, if it's true deep litter we're talking about and not deep bedding - as in, the active, moist, composting reaction - then it might get tricky with the sand, because the mix will need to be moist for the reaction to happen. Moist sand is just a whole separate beast - it might freeze solid in the winter, if the reaction isn't managed well and doesn't produce enough heat. And it might not do well being diluted by a lot of sand.
If we're talking about the deep bedding method though - the dry method that doesn't compost actively - then that would work better, if you want to just put something more comfortable on top of the sand for the chickens to walk on. Deep bedding needs to be changed periodically though (I take mine out twice a year), so when that time comes, it will be harder to do because of the added weight of the sand. Ideally, what I'd do in your situation, is I'd get rid of all the sand, and only do deep bedding with a very thick bed of dry material (pine shavings, hemp, whatever you have). If you want something less cold for the chickens to walk on, and are considering putting that on the floor, there's no reason to keep the sand underneath. It won't help anything and will only be in the way when you need to clean the coop out in the spring (and if you want to compost the soiled bedding, the sand will only be in the way).