If you set up regen braking then the brake line will trigger it as you want for one lever. The other brake lever should be set up to pull the throttle to ground instead, which will not trigger the regen but will cut the throttle.
Best setup IMO, is the regen braking is controlled by a completely separate switch. It can be a horn button type switch. Then brake handles control only mechanical braking, and if you wish, cut the motor power.