It's going to be next to impossible to reroute 50% of your current, no more no less, away from the sensor. Even if you have an identical second wire in parallel and make sure their resistance is the exact same (good luck with your micro-ohm measurement), their impedance will still be different due to the sensor coil present on one side.
Why don't you get a different sensor - say a 160A full scale sensor to replace the 80A one, but now your 80A current comes to half the scale so the controller thinks it's 40A.
Or modify the software that controls the thing (which is something you're bound to do anyway)?