I have been involved in some recent internet discussions about railroad terms, and one conclusion is many of these terms were very imprecise and meant different things to different poeple. To me a doubleheader is two (steam) locomotives coupled at the front of a train. If the second engine is cut into the train, ahead of the caboose, or behind the caboose it is a just a train with a helper (mid-train helper, pusher., banker, etc., etc.) But it would not surprise me that there are some railroaders out there (not to mention railfans) who have used the term more broadly to include any two engine train.