Complex sentence: Six pilots have ejected at speeds exceeding 700 knots (805 mph) and the highest altitude a Martin-Baker seat was deployed at was 57,000 ft (from a Canberra bomber in 1958).
Split into simpler sentences: Six pilots have ejected at speeds exceeding 700 knots (805 mph). The highest altitude at which a Martin-Baker seat was deployed was 57,000 ft (from a Canberra bomber in 1958).