Humans meet some distant alien species. They are mutual first contacts. After learning the languages of each other, they begin to share scientific and engineering knowledge with one another, but here's the problem: given our separate lines of evolution and scientific development, we have different systems of units, computer architectures, character encodings, names for the same mathematical concepts, mathematical notation (including default numeric base), etc...
We have to standardize or coordinate- and this question will begin with units of measure. We can certainly use unit conversions, but this becomes a complicated matter when working on e.g. spacecraft, as past human experience has shown that bad things happen when engineers are using two different units of measure for different components.
The problem here is there are a lot of units and neither species can really strongarm their way into making the other conform to their units (assume both have a well-defined and self-consistent system like SI, so neither is objectively superior). They have to come to a compromise somehow.
How do two independent interstellar species come to agree on a system of units?