Ah crap, cheers I completely misunderstood that. Saved me some more time

That's what

with the same multiplicities

means.

Yes.

Your code is wrong somewhere for that case: a: [2, 2, 3], b: [4, 9, 9].
When so many people passed the Rust kata, please don't post issue but a question, chances are big that your code is wrong. Moreover your posts could be shorter:-)
Please read this: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution + read again the description.

10 / 4 = 2.5 in Python 3.

Although floor division is not necessary (in python) when you divide by an integer:

10 / 4 = 2
10 // 4 = 2
10 / 4.0 = 2.5
10 // 4.0 = 2.0

I consider that to be a mistake in the kata's description; although a British person would call the floors in a two-floor building the "ground floor" and the "first floor," they would still agree with an American that the building had two floors.