The technique used to separate a mixture of substances that are soluble in the same solvent is generally known as chromatography. Chromatography separates components based on differences in their distribution between a mobile phase (solvent) and a stationary phase. Additionally, liquid-liquid extraction (or solvent extraction) is commonly used when substances have different solubilities in two immiscible solvents. However, when substances are soluble in the same single solvent, chromatography is the primary technique because it separates based on differential interactions with the stationary phase rather than just solubility differences. So, the key method to separate substances soluble in the same solvent is chromatography. If more detail or explanation is needed about chromatography or related methods, that can be provided. This answer is based on general separation principles and references from chemistry resources.