"Extreme left" and "extreme right" often do meet when it comes to some things, such as the view that power should be centralised and concentrated. On other topics, such as race, the opinions are usually entirely different.
The concentration of power however always thwarts what was initially sought. Most likely most nazis in the beginning of the movement and most early days communists in Soviet did not want what it eventually ended up being. I dont think its entirely fair to let Hitler and Stalin represent the far right and the far left as they were beyond that and more often acting out of self-interest rather than some kind of ideology.
They have one thing in common though - antisemitism. And not only that, they both claim it isn't antisemitism.
Apparently they both also have:
A hatred for mainstream media (which is why we are on this thread)
An undying love for their dear old mum
An a'hole that points downward