I agree with John on both counts. That the term, like many RR terms, is often used loosely, and that to my mind a "doubleheader" means 2 (steam) engines coupled together at, well, the head.
Sometimes, though, things defy definition. Take this train, run on July 15, '04. This was the first, doublehea..., uh mid-trai..., uh two locomotive consist on the C&TS since 487's rebuilding. It felt more like a doubleheader than a mid-train helper setup, so I called it "an unusual doubleheader" on the website. The flats were dropped off atop the pass.
Your definition may vary.
Linked, rather than embedded for the bandwidth challenged. 'bout 200k each.
Scott