The Loeb Library is among the most comprehensive collections of facing editions of classical Greek and Latin texts. James Loeb was a banker, classical scholar, and philanthropist, famed for the Loeb Classical Library and the Julliard School of Music. It's now possible to search across texts in this online edition, but the small red (Latin) and green (Greek) print volumes have been loved and read by classical scholars for over 100 years.
Dr. John R. Porter, of the University of Saskatchewan, created this collection which I found in Academia.com. He writes, "Part of a repertoire of texts that I plan to employ in a LATN 400 course called “Text as Artifact.” Still in the development stage, but I've begun making them available in case anyone might find them useful, or be willing to pass along corrections, suggestions, etc."