Sauron-Saruman: Why did Tolkien make his two main villains have such similar-reading names?

I often got confused during my very first reading of The Lord of the Rings, because I was mixing-up Sauron with Saruman. 

So how was it that Tolkien allowed his two Big Bad villains to have such similar names - as they appear on the page; and this despite Tolkien being just-about the most deeply-informed and best "namer" among all authors? 

I propose a plausible answer over at the Notion Club Papers blog.