I think part of the problem comes from them being so close syntactically and semantically, it puts the second one you learn in the uncanny valley, just close enough that your brain tries to reuse previous knowledge, just far enough that it can't and you get frustrated. I think that's especially the case if you don't have many languages under your belt.

When going from Python to Haskell, or Ruby to Erlang, you can't get confused between the two.

This is true, I've tried to write Python code in CoffeeScript, for instance using Python's range() or the colon at the end of the beginning of a block.

Python is easy.

Learning easy things and getting used to them makes learning difficult things tougher later.