I used last year a Tetenal's magic-box E6 with three films, by increasing the First Developer time by 15 seconds and the Color Developer's as well as the Blix's time by one minute for second film and +30 s and +2 min. for third film, with success.
I'm willing to use a magic-box C41 with my last three films. Do you think I should add to the Color Developer's time 1 minute for the second film and 2 minutes for the third ?
The thing is, did Tetenal really make a special product for 1 film only? Given their recent adventures, it's highly unlikely and it's just a small size version of their regular C41/E6 chemicals. So, go ahead and find the instructions for the C41 kit, you'll get all the information you need there. BTW, I only mentioned 15" for the CD. Blixing is a process that should be done "to completion" and it won't hurt if you exceed the recommendation.
I have successfully developed 4 120 films (1 Ektar, 2 Gold and 1 Kodacolor II) with these timings :
1. film: as usual (CD is 3:30 with magic-box and 3:15 with Colortec) and BX 4:00
2. film: CD + 15 sec. (3:45), BX + 2 min (6:00)
3. film CD +30 sec (4:00), BX + 6 min (10:00)
4. film CD + 45 sec (4:15), BX +11 min (15:00)