We present a model for convection in a Kelvin-Voigt fluid of order one when the layer is heated from below and simultaneously salted from below, a problem of competitive double diffusion since heating from below promotes instability, but salting from below is stabilizing. The instability surface threshold is calculated and this has a complex shape. The Kelvin-Voigt parameters play an important role in acting as stabilizing agents when the convection is of oscillatory type. Quantitative values of the instability surface are displayed. The nonlinear stability problem is briefly addressed.