Complex sentence: The test marked eight out of ten times the Missile Defense Agency and U.S. Navy's Aegis missile defense system successfully intercepted its target, but was the first time the system knocked out two targets at the same time.
Split into simpler sentences: The test marked eight out of ten times the Missile Defense Agency and U.S. Navy's Aegis missile defense system successfully intercepted its target. But it was the first time the system knocked out two targets at the same time.