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Do people who study bicategories (or double categories) of spans ever use models in which the target of a 2-cell is a string of 1-morphisms, rather than the source, as a way to sidestep coherence issues?
It seems to make sense because the composition of 1-morphisms is defined via a limit, so it's better to map something into it.
Yes, they were used in Dawson-Paré-Pronk's series on the universal property of Spans as freely adding right adjoints, in particular The span construction (section 4) and Paths in double categories (section 2), under the name "oplax double categories", in precisely this context.
Thanks! Section 4 is just the sort of thing I had in mind.