Separate particle detectors measure the quantum states of each particle and send the resulting signal to a coincidence counter. In any experiment studying entanglement, the entangled particles are vastly outnumbered by non-entangled particles which are also detected; patternless noise that drowns out the entangled signal.
Particle detectors for individual particles assess their quantum states and send the results to a coincidence detector. In experiments examining entanglement, the particles in question are greatly outnumbered by unentangled particles, which are also measured; random noise that masks the entangled signal.