Maybe this is a silly question, but ... wouldn't it be better to mention something about being partner cities on the T-shirt?
Here at least, a city can have several partner/sister cities in different countries, but it's seldom publicized at all, except maybe among city officials on the one day that someone like a consul or trade representative comes to visit. It just seems likely to me that a lot of onlookers would have no idea why anyone would be running 'for' Leeds -- they might wonder if Leeds needed help because they had a bad time in the pandemic, or a natural disaster like a flood or a fire, or a lot of poor people, or what.
If you were running to raise money for something in Leeds, like a hospital or a social program or a bonus for Marcelo Bielsa, you could also put that, maybe on the back of the shirt.
Or do some kind of a logo with the names and maybe symbols or key buildings of the two cities, or British and German flags, and the words Partnerstädte, or 'Our partner city,' or '40 years of friendship,' or whatever.
Then you wouldn't have to state the obvious in words: namely, that what you're doing in a race is running. And everyone could still wear the shirt on other cultural-exchange occasions. (-: