If you T in on the low pressure side with a regulated line and do not use CHECK valves then unless the two regulators are EXACTLY the same pressure (And this is not even possible, even if you set them with very accurate test gear because as they cool, it will change, just a little) the lower pressure regulator will vent propane.
This is very likely what happened to the O/P
I have seen it happen, this is why I strongly recommend only the Original Genuine Article.
Oh, by the way. If you hook up an external tank.. odds are it will provide most of the gas during the day. (The Genuine article has Check valves in it).