How would someone with a BA and top grades be compared with someone with, let's say, two BA:s in relevant subjects, but with only average grades?

... assuming both people are equally old of course...

A candidate with one good BA is usually more attractive. Two average BA's looks a bit odd and raises extra questions.For example, two BA's might show a breadth of inability, a lack of ability to think at a more advanced level (why did the candidate not use their extra time at university to progress to an MA or PhD), or poor decision-making (why did the candidate not switch degrees earlier if they chose the wrong subject).It of course depends on what the candidate with one good BA has done with their time that they didn't spend on the second degree. It's always possible to throw away a good academic record with a bad work record (sit on the dole, go to jail, teach).

how about someone finishing two degrees in the same time as one, but with weaker grades? That would probably require as much hard work and dedication (if not more) as focusing on grades, but with a preference for breadth instead of depth...?

I disagree that getting average grades in two subjects shows as much hard work as getting good grades in one. Much as I wish it weren't so, it is possible to get passable grades in most degree subjects at Bachelors level on the back of good A-levels, general knowledge, a bit of cramming and blagging. There isn't much to differentiate the good job candidates in the middle ground of academic grades (which is why the extra-curriculars become so important).There is a lot of flexibility for studying diverse subjects within a single degree (e.g. through the Tripos, module selections, or just taking a Combined Studies degree). It isn't clear why a candidate would need to take two separate degrees to achieve this objective, unless it's a post-decision justification.