The maximum power transfer theorem for DC linear two-ports terminated by uncoupled passive resistors is stated and proved. Unlike all previous works, non-reciprocal two-ports (i.e. two-ports whose resistance matrix is non-symmetrical) are allowed, and power is maximized with respect to the load resistances rather than the load currents. Further, the approach developed is variational instead of direct (i.e. based on the theory of quadratic forms), and thus yields considerable side information on the global behaviour of the circuit. Although calculations are more involved than in the case of one-ports, final results are relatively simple.