The perovskites of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO) were synthesized by the sol-gel method. The two perovskites were mixed into polycrystalline materials, (1 - m)LCMO + mLSMO, where m is the relative weight ratio of LSMO. The compound with m similar to 0.3-0.5 has the lowest semiconductor-metal transition temperature T-p and the largest low-T resistivity. Moving away from this ratio, T-p increases while the low-T resistivity decreases with increasing or decreasing m. Measurements of the magnetoresistance (MR) show that the mixed polycrystals exhibit a sizable MR effect in low magnetic fields which increases upon cooling and reaches its largest value for T --> 0 K. It is also interesting to observe the almost T-independent MR effect over a wide temperature window below 300 K for the mixed materials with smaller m.