In the premiere episode of The Flash (the 2014 CW version), Oliver Queen appears, played by Stephen Amell from Arrow. In the Season 3 premier of Arrow, Barry Allen appears, played by Grant Gustin from The Flash, playing out essentially the same scene from a different perspective. Subsequently, the shows co-exist in the same universe, with frequent references to each others' plots and characters, and even appearances by characters from each others' casts.
As far as I can find, this is the first time that any two DC-based live-action adaptations have crossed over to form a shared universe. Are there any earlier examples of this that I missed?
NOTE: For the purposes of this question, I am using the following definitions:
- live-action adaptation means a live-action television show or movie based on one or more characters that were originally published in DC Comics (or it's legal predecessors, depending on the time frame.) In particular, I'm excluding the animated universes for the sake of my own sanity. (Looking at you, Batman and Scooby Doo).
- crossing over means that characters that are featured in one adaptation appear as the same in-universe character in a different adaptation. This means that "guest star heroes" on shows like, e.g. Smallville do not count as a crossover.
- shared universe means that, following the crossover event, the two (or more) distinct adaptations clearly occur within the same continuity, and events that occur on one adaptation are intended to be occurring simultaneously in all adaptations within the shared universe