They both "dated" Virginia Hill in real life, but I'm not sure that Siegel actually punched out Adonis.
And Dean Jennings, author of the Siegel bio, "We Only Kill Each Other," makes no mention of fisticuffs twixt Bugsy and Adonis. It's one of the many, many embellishments the producers of "Bugsy" added to make the film more entertaining (and it is--it's one of those films that gets better each time I see it).
