The reason some carriers use two sets of codes such as Eastern with the 66L, is to avoid confusion with other flights; i.e. if you've got the EZE586 due at the same time as say, a PGT586, there could be some serious confusion between the pilots and ATC.
On a Wednesday evening, there's an XLA449 goes into NCL at about the same time our OHY449 comes in, I don't think it causes much confusion to ATC as they're usually on different frequencies, but it causes spotters some headaches!!