Abstract: The invention concerns a double-chamber device for mixing at least two viscous substances, comprising: cylindrical and coaxial first and second communicating vessels, each hermetically sealed at one end by a mobile piston, and provided at its second end with a slightly conical bottom end, the two vessels being arranged, preferably vertically, in mirror symmetry relative to a meridian plane perpendicular to their common axis; a central mixing element connecting the two vessels at the two slightly conical bottom ends; and a high pressure hydraulic device for automatically activating the pistons in a coordinated reciprocating movement. A worm screw injection device in communication with one of the vessels or with the mixing element may be used to inject the substances. Mixing of substances may include driving the pistons in a coordinated reciprocal motion to collapse one vessel while expanding the other, forcing the substances through the central mixing element.