Thx for all your replies!!
Finally I decided to make an own implementation on the basis of multizip(...) in the itertools crate from @bluss. I gave it another name (group(...)) because it follows a slightly different use case. For anyone interested in the writing process see this thread.
Here is the resulting code:
https://github.com/Bendrien/ommap/blob/1d9a958077f9b1c3fb035bc2fc6099432d722633/src/iter.rs#L68-L142