I personally use aspen shavings. They are usually not the first recommended because they don't hold humidity, which has been shown important for smooth shell growth (cold+damp=bad, but warm+damp=good), but they are an alternate option for some of the following reasons...if you live in a damp climate the substrate will stay warmer easier, lighter to lug around than moist substrate and sand, and a little "cleaner".
Some people even use dirt.
There are a lot of options. Searching the forum for substrate may give you more information to read through and make a good decision. Most people end up trying out a few things too. For RTs, it's important to have a substrate they can dig into as they are quite the diggers. They like to bury themselves. You probably want at least 4-6 inches deep, depending on the tort's size.
Although I no longer keep russians, when I did I kept them in a 50/50 substrate as well. Russians love to dig so make sure that the substrate is deep enough for them to dig down in. They are also excellent climbers. So, make sure the sides are high enough so that they cannot escape. I also put driftwood and things like that, they love to dig under and hide. Good Luck!