I don't like sand in an enclosure, especially an indoor one....some folks use it....I just have had experience with an impacted tort due to sand and so have decided it does not offer benefit enough to risk the increased impaction risk.....
My post here will bump your thread to the front and you will likely get some others opinions as well...
The more substrate you add, the more warmth it will hold and the better insulated it will be. This also gives you the option of adding a heat cable in the substrate.
You could fill the bottom with a couple inches of sand, since it's cheap and clean, and then throw topsoil/coir on top where the torts roam.
If you are having trouble with temps, definitely close the enclosure, and get something on a thermostat (CHE, heat cable, etc)
I like using sand in my mixture and about 1/3 is a good ratio. you don't want too much of it. it gives the substrate some good drainage; keeps the top layer dry and the bottom layer moist. I've also noticed that it prevents the substrate from clumping up. a really nice soil and sand ratio keeps the substrate "airy". in other words, not compacted. have not had any problems with impaction since I feed my tortoise on a slate rock to avoid getting substrate on its food.
I use the sand in my enclosures and like the way it works. I myself would bump it down to only 1/4 to 1/5 (I prefer the 1/5 myself). A healthy well hydrated tortoise should have no problems with it, even at the percentage your talking of using.