Personally, I don't know where the single-subject concept comes from. If 34 states each have a unique idea for an amendment that they want to share, they should be allowed to call for a proposing convention to let the other 33 hear the idea.
-PJ
As near as I can tell, the source for this error is a 2011 paper by Robert Natelson which Mark Levin cited in his Liberty Amendments.
“The State-Application and Convention Method of Amending the Constitution: The Founding Era Vision” - Thomas M. Cooley Law Review 9.