Sequences in YAML are notably not allowed to be merged. Meanwhile, there seems to be some use-cases where having this capability would be desirable. Also, there are no workarounds in syntax that would enable sequences merging, hence the enabling of flattening sequences of sequences in post parsing would be the way to enable such syntax to work:
tasks:

• !include multiple.tasks.in.one.file.yaml
    - !include different.task.yaml
  The drawback would be the limitation of not being able to use sequences of sequences as a syntax constructor for any YAML Specs feature.