Hi RFsolution,
With line following robots, they have inputs from both (or all) sensors so that they know if they are way too far over.

From your description you have 4 distinct zones of control.
1) the wire is way over to the left of the left most sensor.
2) the wire is between the left and right sensor.
3) the wire is way over to the right of the right most sensor.
4) their is no wire

If you use a differential output from a comparator or such then you could loose the ability to sense if the wire goes outside the two sensors (far left and far right).
However, if you ADC scanned both sensors at a rapid pace then your robot is less likely to get lost also, it means that your control loop can be tuned so that the robot can speed up when it the wire is in the centre and slow down if it thinks it's loosing the wire. But also, the adjustments to motor control can be small when the wire is in the centre and progressively coarser the more the wire moves from the centre.

Also, you don't necessarily need to use coils of wire as detectors, the analogue hall effect sensors would be an easier, smaller and cheaper option. Be aware though that Hall effect sensors come in at-least two types, one has a digital output that is on when a certain field strength is reached, these types will not work for your needs. However the analogue ones give you an output that can be fed directly into the ADC to provide a voltage based upon how much of a magnetic field is being picked up.

I hope this helps...