Responses
There are some that do both genre, but use different pen names like TAKAGI Shigeyoshi/OGAWA Chise or UKYOU Ayane/SAKYO Aya (allthough both have started with shoujo, but in recent years have only published BL mangas)
NAKAMURA Asumiko does all kinds of genres
There are a lot of yaoi artist that definitely would make better if not best shoujo/seinen/joesi manga. Like Kizu Natsuki, Watanabe Asia, Zaria/zariya, hiwatari haruki, bubunhaten/kazuki rai, ichikawa ichi.. who do you think would make an absolutely wonderful manga of different genre other than yaoi if given a chance to make one? Like Yamamoto Kotetsuko, I really, really adore Honey Sweet Kitchen (σ≧▽≦)σ and yoneda kou's new josei manga..