I think that it isn't a "myth" per se; it seems to be an ideal maybe Disney aspires to, but it certainly is no rule. Just about every character that has a ride has a M&G in the same park that would put them in two places at once, without even thinking about characters that appear at multiple parks. Let's see:
-Buzz is in his ride and then outside in Tomorrowland
-Pooh characters meet right outside their ride
-Buzz and Woody meet right across from TSMM in DHS
-Parade Mickey goes right by the Town Square Theater M&G
-Mickey is also in Philharmagic while he is in Town Square, the parade and/or the castle show, although this could be a duplicate Waldo from Muppetvision instead of a duplicate Mickey
The strangest one that I just thought of is Donald at Epcot. There is a ride where the whole point is that we can't find Donald when he can regularly be found at the M&G outside. Why didn't Jose and Panchito just look there instead of travelling all around Mexico (it's a big country)? They must not have picked up a Times Guide on their way into the park. You can also meet Donald all day at the Character Spot.
I'm not complaining. I think it's a courtesy extended to the guests to give them access to the characters that they want to see and to have them in logical places (near their rides). Courtesy comes before Show. Although Mickey in MK probably pushes the limits of what's acceptable.
