Primary solar wind protons do not cool off adiabatically with distance, but appear to be heated. Also secondary protons, comoving with the solar wind as pick-up ions, appear to behave quasi-isothermal at their motion outwards to the outer heliosphere. In a synoptic view presented here we see these two phenomena as closely related. This is proven by solving a coupled set of enthalpy flow conservation equations for the two-fluid solar wind system consisting of primary and secondary protons. The coupling of these equations comes by the relevant heat sources, namely by dissipation of MHD turbulence power to both ion species. We take into, account the dissipation of both convected turbulences and turbulences locally driven by newly injected pick-up ions. Initial conversion of free kinetic energy of freshly injected secondary ions into turbulence power is followed by partial reabsorption of this energy both by primary and secondary ions. We integrate the coupled set of differential two-fluid equations, study the primary proton temperature within this two-fluid context and find a non-adiabatic behaviour with radially and latitudinally variable polytropic indices. Inspecting latitudinally variable solar wind conditions as found from observations by McComas et al. (2000) we predict latitudinal variations of primary proton temperatures and show that the secondary proton temperature with increasing radial distance asymptotically attains a constant value with a magnitude essentially determined by the actual solar wind velocity. From that we also conclude higher pick-up ion temperatures and pressures at higher latitudes.