A method and high-pressure mixing device for co-injection of polymeric reactive components, in particular for polyurethane and epoxy mixtures. The polymeric components are supplied in a common pressure chamber where they flow at a same pressure and in an unmixed state into a forwardly converging fore portion of the pressure chamber, and through a settable co-injection orifice to be co-injected, in the unmixed state, into a mixing chamber transversely oriented to the pressure chamber. The settable co-injection orifice consists in an elongated restriction that longitudinally extends on a side wall of the mixing chamber orthogonally oriented to an intersecting the forwardly converging fore portion of the pressure chamber; a first cleaning member and a second cleaning member are sequentially reciprocable in the pressure chamber to eject the remaining unmixed polymeric components, respectively in the mixing chamber to eject the remaining mixture, and stop elements are provided to set an open section of the elongated restriction, by adjustably stopping the fore end of the cleaning member for the mixing chamber, in respect to the same elongated restriction of the co-injection orifice.