The lone 5 and 6 car set in Corona is 7866-7870 and 7860-7865, which might have been paired already, I haven't been there in awhile. 7866-7870 is CBTC testing, not sure about 7860-7865. There have been some sets that have been decoupled in the past (I beleive it was 7331-7335 which was decoupled from 7336-7340) but they recoupled the set back together. But knowing the MTA, they will start mismatching the sets soon, unfortunately.