Lots of words start out as two and become one. Currently "eachother" would be considered to be two words, but maybe in time they will be one. "Today" used to be hyphenated as "to-day," which was probably a contraction of "to this day." Or take the days of the week: "Thursday" once was "Thor's day," now it's just hump day (or is it humpday?). 

Riiiiight. 
 
Next you'll be trying to tell us that the name of Monday is derived from Old English Mōnandæg and Middle English Monenday, which means "moon day."

Weds day is hump day to me.

prolly was my last time.
 

fnar fnar I make joke
 

i larf at me joke

To me, the question of writing words combined or separately is one of the most puzzling things in the English language (having practically no formal education in the language), and I quite often misunderstand sentences due to that. In Finnish it's an essential thing in avoiding misunderstandings and we have a lot of combined, long words, and thus very exact and unambiguous expression in general.
Maybe not as long and complicated as the German language, though. I remember how impressed I was seeing
a "Lebensmittelwissenschaftlerin" (nutrition scientist) interwieved in a german news bulletin, but I know that ain't the "worst".
Manic Street Preachers... are they manic streetpreachers or the preachers of Manic Street?
Pike perch, pike-perch or pikeperch? You can see all versions used.
Tontos Expanding Head Band was once hilarious to spot.

Eye laf wit u.

Health care is now healthcare, so should I do not care now be Idonotcare?

Digital sensations isn't what it's cracked up to be - Proctologist