If two solutions with different densities are placed in a glass, and they form two layers, does this mean they are not soluble?
They are not mutually soluble. Solvents that form two layers are called
Hexanes and water, diethyl ether and water, methylene chloride and water, are IMMISCIBLE. There are many other examples. They form 2 separate layers, with the most dense solvent on the bottom. (There will be some water dissolved in the so-called organic layer, and some organic solvent in the water layer, but there will still be 2 bulk phases).
In the laboratory, often a solute is "partitioned" across 2 such phases. Organic solutes are collected in the organic layer, and inorganic solutes (ions etc.) are collected in the aqueous layer. Such a process is called "solvent extraction".