Unusual splits
I have actually been doing my Masters research on those types of combinations. We have a school near here that ALL the classes have unique splits. The entire school has splits such as 2/6, 3/5. There are a lot of benefits to that type of combination. It is not as hard as people think, you just find commonalities between the two grades.
The idea of these classrooms are grounded in research on child development and learning approaches that recognize that children’s cognitive, physical, social, and emotional abilities will emerge on a timeline that is unique for each child and that these abilities will not always develop in the same sequence or at the same rate.
This combination has positive characteristics that relate to students' social skill development; it provides opportunities for enhancement of learning in the lower grade level group through exposure to upper grade level work, it offers reinforcement of earlier learning for upper grade level students, and it provides opportunities for children to learn through peer tutoring.
The research also shows that students in multi-grade classrooms show increased self esteem, more cooperative behavior, better attitudes toward school in general, increased pro-social (caring, tolerant, patient, supportive) behavior, enriched personal responsibility, and a decline in discipline problems.
I have been researching this for over 4 years, I envy you. I dream of teaching a class like that!!!