Magnetic properties of stage 2 Co_cNi_1-cCl₂ graphite intercalation compounds (GICs) for 0<or=c<or=1 have been studied by DC magnetic susceptibility. These compounds can be considered to approximate two-dimensional randomly mixed ferromagnets with XY spin symmetry. As a function of Co concentration c the average effective magnetic moment P_eff, and the Curie-Weiss temperature Theta have been determined. The measurements indicate that Co²⁺ and Ni²⁺ spins are distributed randomly on triangular lattice sites of each intercalate layer. It is found that the intraplanar exchange interaction J(Co-Ni) between the different spins is larger than that between like spins, J(Co-Co) or J(Ni-Ni):J(Co-Ni):=1.2(J(Co-Co)J(Ni-Ni))¹2/. As a function of concentration, the susceptibility shows a rounding effect at the magnetic phase transition for a series of compounds with c=0.1, 0.19, 0.35, 0.40, 0.52, 0.63, 0.80 and 1.0. The authors have assumed a Gaussian distribution of transition temperatures to determine the average value of transition temperature (T_c), the distribution of transition temperature sigma and the critical exponent beta . It appears that the critical exponent beta is unchanged for 0.35<or=c<or=1.0: beta =0.09+or-0.01. They obtain for the initial slope of (T_c) versus c at c=0 the value -1.21+or-0.02. The ratio sigma /(T_c) shows a broad peak centred at c=0.5 which is explained here qualitatively by both finite size effect and macroscopic Co concentration gradient within the intercalate layer.