A crop cutting apparatus comprising: a frame structure arranged for forward travel over ground having a standing crop thereon; a cutter bar secured to the frame structure and extending transversely across a front end of said frame structure; a plurality of knife guards mounted in spaced relation along said cutter bar and projecting forwardly therefrom; each of said guards having a ledger surface with opposed side edges (15A, 15B) thereof arranged to provide first and second shearing edges; a first and a second sickle bar (24A, 24B); the first and second sickle bars (24A, 24B) being mounted in transversely extending position and being driven for reciprocating movement relative to said knife guards such that the first and second sickle bars reciprocate at the same frequency but in opposed phase; the first and second sickle bars (24A, 24B) each having a plurality of knife blades mounted thereon for movement therewith with each of the knife blades having two side cutting edges (23F, 23G) which cooperate with said shearing edges of said knife guards; first and second drive systems (36, 37) each for driving a respective one of the first and second sickle bars through repeated cycles of reciprocating movement where each cycle extends from one end position of a stroke of the respective sickle bar up to an opposed end position of a stroke of the respective sickle bar and back to said one end position; each of the first and second drive systems (36, 37) including a sensing system which generates a plurality of sensor signals for each cycle of reciprocating movement; and a control device responsive to the sensor signals from both of the first and second drive systems (36, 37) for advancing or retarding one of the first and second drive systems (36, 37) so that a number of sensor signals obtained from the first drive system is continually maintained so as to be substantially equal to a number of sensor signals obtained from the second drive system. a frame structure arranged for forward travel over ground having a standing crop thereon; a cutter bar secured to the frame structure and extending transversely across a front end of said frame structure; a plurality of knife guards mounted in spaced relation along said cutter bar and projecting forwardly therefrom; each of said guards having a ledger surface with opposed side edges (15A, 15B) thereof arranged to provide first and second shearing edges; a first and a second sickle bar (24A, 24B); the first and second sickle bars (24A, 24B) being mounted in transversely extending position and being driven for reciprocating movement relative to said knife guards such that the first and second sickle bars reciprocate at the same frequency but in opposed phase; the first and second sickle bars (24A, 24B) each having a plurality of knife blades mounted thereon for movement therewith with each of the knife blades having two side cutting edges (23F, 23G) which cooperate with said shearing edges of said knife guards; first and second drive systems (36, 37) each for driving a respective one of the first and second sickle bars through repeated cycles of reciprocating movement where each cycle extends from one end position of a stroke of the respective sickle bar up to an opposed end position of a stroke of the respective sickle bar and back to said one end position; each of the first and second drive systems (36, 37) including a sensing system which generates a plurality of sensor signals for each cycle of reciprocating movement; and a control device responsive to the sensor signals from both of the first and second drive systems (36, 37) for advancing or retarding one of the first and second drive systems (36, 37) so that a number of sensor signals obtained from the first drive system is continually maintained so as to be substantially equal to a number of sensor signals obtained from the second drive system.