Complex sentence: When ERS-2 was launched ERS-1 shared the same orbital plane allowing a Tandem mission, with ERS-2 passing the same point on the ground 1 day later than ERS-1.
Split into simpler sentences: When ERS-2 was launched, ERS-1 shared the same orbital plane. This allowed a tandem mission, with ERS-2 passing the same point on the ground 1 day later than ERS-1.