Reason for two springs in the exhaust valve/ concentric spring fitted in ae cylinder valves

1. If frequency of natural vibration of air or exhaust valve springs is a harmonic of camshaft speed,the spring may vibrate axially(termed surging), it can be corrected by arraying spring in pairs or modifying size

2. To prevent damage if the spring falls, the other may remain as backup preventing the spring from falling on to the piston.
3. Total stiffness and thickness requirement in designing safety requirements can be reduced.
4. Uses less space in comparison