Not with both arms at 6 o'clock, but another crankset where the arms are not locked at 180 degrees apart is
Rotor Cranks . Rotors are cammed. With Rotors, the pedal coming back up goes past the top before the pedal going down reaches the bottom. Likewise, when the next pedal comes up, it too goes past the top before the other one gets to the bottom. So there is never a spot where both of them are at a dead spot in the stroke.