Are you sure the drive won't read a DL disk? As far as I know, dual layer has been a feature of DVD from day one. It's possible that an old drive has partially failed and won't read DL disks. Or it could be like some Toshiba laptop drives which are fussy about the brands they'll read (Verbatim good, "Maxell" not good). Or there's some file system issue, like the disc is UDF and the computer doesn't support that. Or it's running an ancient Windoze that doesn't understand DVD at all.
Anyway, getting Imgburn to automagically span data across discs ain't gonna happen. The closest you could come to that is to use an archive program (WinRar, WinZip, 7zip) to store the image (or data) in a multipart archive file. Then burn the chunks onto two separate 4.7. A more reasonable solution is to divide up the data so it's in two different folders, then burn those.