We prove that, for positive integers a, b, c and d with c = d, a > 1, b > 1, the number of simultaneous solutions in positive integers to ax 2 − cz 2 = 1, by 2 − dz 2 = 1 is at most two. This result is the best possible one. We prove a similar result for the system of equations x 2 − ay 2 = 1, z 2 − bx 2 = 1.