E-44 freight locomotives with 2 half pans next to each other usually used the "fronter" pan with the "knuckle" leading.
Many Great Northern locos had bus bars, but PRR removed them from the ones they bought in 1958. ConRail didn't use the bus wires on the E-33s, although New Have had when they were EF-4s, and ran with one pan per pair of locos.
Old Reading cars had bus bars, a RDG press release cited 2 advantages: less wear due to less raised pans needed, and less arcing if more than one was up, since they wouldn't always try to arc at once. (If one bounced or ran under dirty wire, another might still make good contact and power the train.)
Some pairs of Metroliners had bus wires so only one pan would be needed. There were special rules at Penn Station, New York, that the other pan must go up before the 1st pan went down, unless the wire was dead. Possibly they were afraid of unquenchable arcs under the low wire? I don't know what happened with single cars, or cars without jumpers, the rule didn't apply. Some Harrisburg Capitoliners were 3 car trains.