GOSH you saw it on Sky news. Now I'm impressed - I was sure it was the Daily Mail

When you, or anyone else says "specifically allows" it HAS to be written. that is what the word "specifically" means; that it has been specified in a manner that is clear and precise.

NOT fill in the Blanks.
NOT heard on Sky news.
NOT if you look at it through a polarizing filter under Green Light

Article 50 does not say that negotiations can take place in parallel. 

It says the rules are what the council of Europe say they are, not what you want them to be, and the Council of Europe has said no parallel negotiations.