This paper presents a detailed experimental investigation of the nonlinear dynamics of a dual-beam optically injected semiconductor laser. Unlike previous works, we focus on the situation where the power and the frequency of the injection light wave are fixed and the free running frequency of the laser subject to optical injection is in between the two injection beams that have a large frequency separation. The nonlinear dynamics of the slave laser (SL), i.e., the laser subject to optical injection, for different SL forward bias currents are examined. Results show, in addition to the regenerated injection light wave, a fundamental oscillation together with its harmonics are generated when the SL interacts with its nearby injection beam. The SL output frequency components are stabilised when one of the harmonics of the fundamental oscillation is located at the injection beam that is far away from the free running SL. More than six-fold improvement in the frequency stability of the resultant output microwave signal, compared to that generated by optical mixing, is obtained. Experimental results also show the SL nonlinear dynamics have different behaviours when the two injection beams have different frequency separations.