How do humans learn to combine tasks?
The term I was looking for is "concurrent activities". Some research in the domain of hierarchical learning has been done in this domain by Rohanimanesh and Mahadevan.

According to this literature review on hierarchical learning, basically what they did was determine how multiple tasks can be managed without interfering with one another and how they should be scheduled.

I couldn't find any cognitive models of this concurrency, so there appears to be quite a bit of work done on the idea of multi-tasking, which might be synonymous. Unfortunately, most of this seems to be quite firmly in the camp of the Classical Cognitive Science Paradigm (such as the work by Niels Taatgen which interestingly rejects hierarchy) with not a lot of thought put into neurobiological plausibility. Additional answers pointing to fMRI experimental data similar to the one found here for hierarchical learning would be greatly appreciated.