A little overlap in throttle and braking is acceptable. Keep in mind that for a kart to work properly, it has to flex and have some kind of torsional load through the chassis to jack weight. This means there always has to be some sort of load occurring, ie; no coasting. There should always be either some throttle or some braking happening to keep the kart ‘sprung’. This can cause some overlap in the pedals as you modulate brake and throttle. However, riding the brake and throttle can wear out clutches, brakes etc. like everyone else mentioned so any overlap in pedal input should be minimal.