You want a circuit that gives more power to the left motor (than the right motor) when the right line detector sees the line (to steer it back because it has gone too far left). And another circuit (identical to the first one) that gives more power to the right motor (than the left motor) when the left line detector sees the line).
If it is on top of the line, neitehr line detector should see anything because the line is in between both sensors.
THe simplest circuit to do this is to have a transistor controllign current flow through the motor and having a line sensor (a black/white sensor probably with an illumination LED) driving the gate of the motor transistor. Only one motor would ever be on, and the robot would follow the line in a strange zig zag manner.