Electrical power for the train

When a train has two locomotives, from which unit is the head-end power for the train being drawn from?

Either. For whatever reason, sometimes the lead unit is in HEP and the trailing unit pulling, and sometimes the second unit is in HEP and the lead unit is pulling.
Sometimes, they are both running. Usually the second unit is in HEP though, that way the 2nd unit isn't pulling the train and pushing the lead loco, causing (possibly) some bumps for the crew.

The last time I've seen two Metra's double headed, it was two F40PHM-2's. When leaving Joliet Station, they were both powered. I've also seen them when the first is powered, and the second is in HEP, and when the first engine is in HEP, and the second one is doing all the work.

Guys,

When you are running a multiple-unit set on a passenger train, all units work...At least in Metras case. Say you have a dubbleheader with 146 and 149. 146 is the lead unit; 149 is in HEP. When you notch up on the throttle both 146 and 149 would start to provide power, even though 149 is in HEP. It can be done though where the unit providing HEP is the only one working; the engineer could isolate 146 so as to have piece and quiet on the ride out. Amtrak is also known for doing this on the Southwest Chief; and any of the abuv configurations of engines, as well as lead unit providing HEP but both working, can be done with ANY PASSENGER UNIT. And no, this isn't the place to get into the technicalities of freight and passenger units working together...

I have noticed on RI it is usually the lead loco providing the HEP. Last weekend I must have seen 212 & 204 four times and if I remember correctly..212 always provided the HEP.

I saw it the same day you did, Kyle. 212 was providing the HEP. That was the first train I saw when I got to Joliet.