Not only do they both star a prepubescent girl who’s supposedly very powerful, but her power hasn’t been fully awaken yet, they also feature a clueless guy who basically follows her orders. And of course they both somehow fall in love with each other, but have a hard time expressing so.
Anyway, I did some extra research and it turns out the voice actors for both the main female and male characters are the same!
Hino Satoshi (日野聡) voices both Hiraga Saito (from Zero no Tsukaima) and Sakai Yuji (from Shakugan no Shana).
Kugimiya Rie (釘宮 理恵) voices both Louise (from Zero no Tsukaima) and Shana (from Shakugan no Shana).
If I had to guess, the story/manga is probably done by the same artist/company too.