Sand? Fine gravel?
Honestly I never understood the need to have 2 sorts of substrate, it's gonna mix over time anyway.
If you want to use it for plants, either use regular soil with a cap, use an aquasoil, or to save money without having the potential of a muddy mess, use safeTsorb or Turface.
If not plants, just use sand or gravel.
You aren't likely to gain anything by using reptile substrate, just the potential for a mess.