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No, they can't?!
Or how are you supposed to do?

I had a leader that was a builder.
I had a worker building a world wonder.
It was a few years left, so I tried putting another worker on the same tile to help out.
And I didn't see any button or option to help building it to speed up the work.

What am I missing?

Can't try again sadly, RIP young leader ;D
This question / problem has been solved by Ferashyimage
There is no button, you just put the worker ontop of the other worker and it will triger the Builder's bonus.
Oh, that could have been made more obvious, but good to know, thanks :)