I like the PP's diagram. I've only taught a combination math class for two years at the 3rd/4th grade level and looped with the one group the second year. (I was working on my Masters and thought teaching the little kids would be easier than the middle schoolers.
Boy, was that a total misconception!!!)
My one suggestion is that you alternate daily which group you begin with direct instruction. This would help with balancing the time between the two groups. I was stressed when I first started a combination math class - there never seemed to be enough minutes to cover everything. I was surprised to learn, however, how much each group learned from the other, especially the advanced and lowest learners. You can do a lot of differentiation in a mixed class.
Once the kids have a routine it goes quite smoothly. Lots of paperwork however so get your procedures for grading, etc. firmly established early or you'll get overwhelmed especially with a class of 29. Remember its not just one set of math papers to correct daily - it will be two sets.