The finner sand will work its way to the bottom ant the larger crushed shell to the top.
Thats correct and the two will eventually be mixed or settled into each other. The good news though is that you can use crushed oyster shells as a substrate. Its dirt cheap as a filler/substrate and works just fine for long term. Thats all I used to use and the only reason I didn't use it this time is that I didn't know where to get it around here. It used to cost 80 cents for a 50 pound bag and is used in chicken feeders. Since I live in cattle country, there just wasn't any here. If you can tell me where to get big bags of crushed oyster shells, I would appreciate it. If used as the only substrate, it tends to settle some and pack real hard so its difficult for diggers to dig it up and make a mess. And as Gary says, sand sifters wont do well in it. It will grow pods and bristle worms like craxy though.
Make sure to lok through it and wash it well so you get all the trash out of it, but, this is true of any substrate.