Complex sentence: There were two research groups that each played extremely important roles in the early development of Russian Cold War era jet propulsion; the Leningrad Gas Dynamics Laboratory (the GDL), and the Group for Research of Reactive Propulsion (the GIRD).
Split into simpler sentences: There were two research groups that each played extremely important roles in the early development of Russian Cold War era jet propulsion. The first was the Leningrad Gas Dynamics Laboratory (the GDL) and the second was the Group for Research of Reactive Propulsion (the GIRD).