They worked together in Sumomo mo Momo mo and were co-winners as best young seiyuu in the Seiyuu Awards for 2006. And they seem to have become friends. They had lunch together recently -- after Aaya had finished a Lucky Star recording session -- and both posted in their blogs how much they enjoyed each other.
Aaya said Yui was extremely funny -- "but of course she was drunk" (she wasn't). Yui said Aaya is so funny they laughed the whole time. Yui even referred to Aya as "Hirano sensei," which is what you would call a teacher or the master of an art (e.g., a senior seiyuu or a mangaka). Aaya referred to Yui as "Kano," and "Yui-san."
(l-r) "The girl who ate the cellphone" (Yui from Aaya's blog; probably a play on words from the animated film The Girl Who Leapt Through Time). Aya from Yui's blog. Note that Aaya has a phone that takes bigger and clearer pics. Despite the quality, the shot of Aaya is one of the nicest I've seen.