Why not who?

[v0031]

George Stephenson was an engineer who is considered to be the father of the railways for his major innovations that laid the groundwork for railway transportation. He’s widely known for having established the "Stephenson gauge," which is the standard railway track gauge used by most railway lines in the world. But just as importantly, he’s also the father of Robert Stephenson, who himself has been called the greatest engineer of the 19th century.
In 1825, the father and son duo, whom together founded Robert Stephenson and Company, successfully operated the Locomotion No. 1, the first steam locomotive to carry passengers on a public rail line. On a late fall day in September, the train hauled passengers on the Stockton and Darlington Railway in north-east England.
As a major railway pioneer, George Stephenson built some of the earliest and innovative railways, including the Hetton colliery railway, the first railway that didn’t use animal power, the Stockton and Darlington Railway and the Liverpool and Manchester Railway.
Meanwhile, Robert Stephenson would build upon his father’s achievements by designing many major railways throughout the world. In Great Britain, Robert Stephenson was involved in the construction of a third of the country's railway system. He also built railways in countries such as Belgium, Norway, Egypt and France.

I thinks the red should be who.

[🦀🎂🦀🎂]

Yes, you're right; the "whom" is incorrect because it is the subject of the verb "founded". They probably got confused because it's in a subordinate clause?

[Anzeigenhauptmeister]

Yes, you're right; the "whom" is incorrect because it is the subject of the verb "founded". They probably got confused because it's in a subordinate clause?

Or they wanted to write both of whom, which would also explain the comma.

[🐒Gods of Prosperity🔱🐲💸]

Maybe they thought "whom" was the plural of "who."