Shouldn't that just be:
"Part of a Multiple-Unit locomotive."
or
"Part of a Multiple-Unit locomotive consisting of "..loco1.backer_name.." "..loco2.backer_name
"Consist" (pronounced CON-sist) is the technical railroad term for a linked group of locomotives or rolling stock. I wanted to use realistic terminology.
After a little searching, I found that British railways also use the term "multiple working" as a verb, and "multiple-unit" refers usually to self-powered passenger cars. I'll think about what is most clear.