What two countries contained half of the world's 10 largest cities by 1900?

The two countries were great Britain and the united states

USA had a number of such cities such as New York, Philadelphia while UK boasted of cities such as London.

the reason why these countries had such a high number of developed and populated cities is that they were centers of industrialization, with Great Britain being the first country to industrialize. Cities in America also grew in size after the industrial revolution of 1870s, and an increased number of immigrant begun to settle in the country.