Two at a time!!

I have been asked for a long time for a book that teaches, or "Introduces" two instruments simultaneously, like two Razzamajazz books rolled into one.
I am finally doing this, and book one for flute and clarinet is nearly finished. The idea is, that piece one and two just use the first notes the two instruments learn as a duet. The next pieces use the first two notes for both instruments then the first three, and so on up to ten notes. Its been quite a challenge, as the notes learned don't relate that easily to start with. I think I've pulled it off,but its been like doing a puzzle, but has demanded some interesting harmonic backing.