Postby blkmage » Tue Jan 12, 2010 2:46 pm
I've mentioned this in another post, but each series pretty much came up with their respective characters independently. Both the Lucky Star manga and the Clannad visual novel were started/released within a few months of each other back in 2004, which makes it incredibly unlikely that there was any inspiration for either of the characters. Since then, Clannad was hailed as an amazing visual novel, while Lucky Star got huge with its anime adaptation in 2007.
What is likely to have happened is that Clannad got really huge, especially among otaku, and the mangaka for Lucky Star realized Tsukasa and Kagami were incredibly similar to Kyou and Ryou and that made for a few convenient jokes and references. It almost certainly wasn't planned, since Lucky Star got serialized in January 2004 while the Clannad PC game came out in April of the same year.
Of course, the similarities end with their personalities and appearances. Their development in their respective stories is really, really different, especially to those who are familiar with the Clannad visual novel (since Ryou's and Kyou's arc never got animated in full).