Not completely clear cut, I have found.
A Web source I found, The Mayfield Handbook Of Scientific and technical Writing,
http://www.imoat.net/handbook/everyone.htm, says:
Everyone refers to all the members of a group of people. Every one refers to each member of a group of items or people considered individually.
Turning to some books I have here at home,
Line By Line: How To Edit Your Own Writing by Claire Kehrwald Cook, Houghton Mifflin, 1985 says:
Everyone is a pronoun. Every one is a phrase made up of the adjective every and the pronoun one. The one- and two-word forms are not interchangeable. Use everyone only where you can substitute everybody.
The Writer's Harbrace Handbook, by By Cheryl Glenn, Robert K. Miller, Suzanne Strobeck Webb, Thomas Wadsworth, 2004, says:
"Everyone" means "all": Everyone should attend, "Every one" refers to each person or item in a group. Every one [of you] should attend.
However, "International English Usage", by Loreto Todd and Ian Hancock, Routledge, 1986, differs in that it claims that "every one" only applies to inanimate objects and animals.
So you pays your money and you takes your choice.