...A double bill of Les Visiteurs(1993) and Les Visiteurs 2(1998) is available on one two-sided French R2/PAL DVD - 2.35:1 anamorphic, optional French, English & Arabic subtitles... http://www.dvdfr.com/dvd/fiche.php3?id=1547
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I bought the double bill DVD while looking for the first movie in a Paris store (it wasn't available in the UK at that time). However, I discovered it almost by accident - the spine only indicates the second movie (the first movie is included "en cadeau").
The second movie ties up the loose ends from the first movie. I don't know if the sequel was pre-planned but the loose ends were certainly glaring enough to suggest that it might have been.
I thought that the first movie was the funnier and more satisfying of the two, but if you're a particular fan of the first then it's probably worth seeing both.
For any UK resident who hasn't seen either movie but who wishes to buy the first one "on spec" then it's certainly worth paying slightly more to import the French disc.