Upper Harlem - FL9's - one or two ?

Lately, they seem to be using FL9's very frequently on the run between Wassaic and Southeast. Sometimes there is one FL9, and sometimes two, back to back. What determines if they use one or two ?

On a recent trip back from GCT, the conductor made the following announcement at Southeast: "This is the last stop on this train. You do not have to go home, but you can't stay here. Have a nice day !"

The need for two is if one of units has trouble like with HEP and other has other trouble.
there is absolutly no need for two units if the engine is 100% ok.

Latly the harlem line has been running single fl9m,s the last time i saw a pair of fl9m,s on the upper harlem was on september 5th 2008 but since than latly ive been seeing single fl9m,s.

There's no rules. There no rhyme or reason. They just assign whatever servicable power they have on hand to pull a train.