Complex sentence: Ney joined Frank Oppenheimer and Edward J. Lofgren, who both had arrived in Minnesota about a year earlier, in attempts to use balloons to study primary cosmic rays at the top of the atmosphere.
Split into simpler sentences: Ney joined Frank Oppenheimer and Edward J. Lofgren, who had both arrived in Minnesota about a year earlier. These three began to use balloons to study primary cosmic rays at the top of the atmosphere.