Here's a question:
On Amtrak trains which have two locomotives, why do some have the secondary one facing forward, while others have it facing backward?
Also, two locomotives is called "double-heading", right?
It's completely the option of the dispatching yard. The way the locomotive controls are wired, it will run just the same if it is facing forward as if it is facing rearward. The forward/rearward (as in the classic A-B-B-A lashup) was favored during the streamliner era for symmetry, but forward/forward is more practical from the standpoint of keeping the train operable and moving if the "point" locomotive goes bad-order en route.
"Double-heading" was a steam term for operating two locomotives on the same train for much or all of the run, as opposed to a "helper" locomotive which was added only for a short run up a heavy grade. Double-heading required two full locomotive crews as steam locomotives were not able to be fitted with MU (Multiple Unit) controls. Edit To Add: Coordinating the work of the two locomotives was tricky and, in the days before widespread availability of reliable radios, usually required whistle signals.