Not really the boxset thing, but David E. Kelley did this a couple times with Ally McBeal, The Practice, and Boston Public. Part 1 of an episode would happen in one show, and then Part 2 would happen in the other show. It was kind of annoying, actually, as it assumed you actually regularly watched all of his shows.
Boston Legal, too. Besides being an outright spin-off of The Practice, there was at least one crossover from Boston Public (the principal, whose name I forget, made an appearance on Boston Legal).