Well, we're talking about population-level health here, not individuals. And, in general, places where it's easier to get around using your own body have healthier people. Our current tax policy actively disincentivizes this - literally collecting _less_ tax could result in better health outcomes if it meant narrower, safer streets (which are cheaper to build and maintain), less parking (parking spots can cost tens of thousands of USD/EUR depending on where they are, and more insidiously, often displace housing), etc.
Public policies are designed to work on populations, massive number of people - not anecdotal evidences like these.
And taxing private transportation / providing incentives for public transportation does not only benefit our health, it also bring other benefits; global warming issues, dependency on fossil fuels, etc
From the health perspective I agree, but there's a pretty clear reason to tax the second person more, since they have a car: more emissions, road use, infrastructure costs etc.
You can have a sedentary person who works from home and orders takeaways.
You can have a person who has a car who goes to gym 4 times per week, hikes during weekends and plays tennis 2 times per week and eats healthy food.
There is no reason to tax the second person more than the first one.