Lucky Luke by Jean Giraud & Blueberry (sort of) by Morris.
In 1971 the legendary French comic magazine Pilote thought it would be cool to have their two best know western strips switch artists. Most of you probably know Jean ‘Moebius’ Giraud, who was the artist drawing Blueberry. Morris, who drew the strip Lucky Luke (written by René Goscinny of Asterix fame), is less well known outside of Europe. (which is a shame the man was a great cartoon artist and the original series of Lucky Luke is a European classic)
The top image shows a Lucky Luke page by Morris redrawn by Giraud, the bottom image is a Blueberry page originally drawn by Giraud and reimagined with Lucky Luke characters by Morris.
I found a smaller version of the original French version here. These larger images are from the Dutch translation.