How to perform a merge between two large datasets using mapreduce

I'm comfortable the mapreduce paradigm and would like to use datasets and matlab's mapreduce functionality to merge two modestly large datasets (10's of gigabytes).

After reading through the documentation though, it's not obvious to me how to map 2 different datasets that require different map functions, into one dataset.

That sure seems like a canonical use case, I'm surprised it's not mentioned clearly in the documentation.

Is there a straight forward way of approaching a large merge?