For those unfamiliar with either, Ikki Tousen tells the tale of famous Sangoku era figures being reincarnated as busty badass high school girls (and fighting), while Koihime Musou revolves around moe-er versions of the Sangoku characters in their relatively appropriate setting (who fight). Both series are starting new TV anime seasons this April: Ikki Tousen XTREME XECUTOR (season 4) and Shin Koihime Musou ~Otome Tairan~ (season 3).
The collaboration isn't anything as cool as an anime crossover or anything, but the two series' Internet radio shows will apparently swap guests, and there'll be some kind of joint event at Tokyo Anime Fair this March. No, the main reason I'm posting about this is...it just makes sense. So much so that I wonder if a collaborative OVA might not work out. You know, kinda like the Lupan vs Conan special!
But I'd still prefer to see Sengoku Basara get thrown into the mix. Totally different eras, mind (Era of the Three Kingdoms: 2nd-3rd century AD; Sengoku period: 15th-17th century), but how cool would that be?