Sith Lordship and Knighthood Under The Rule of Two

You only Become Sith by Killing a Sith - in Cold Blood.

Or trying to.

This Does Not However Mark the Start of Your Formal Training...

But you are now an Apprentice.

Whether you know it or not... 

Sith Knighthood and Apprenticeship Exists as a 

Progressive Series of Contracts 

Between Master and Applicant/Apprentice

From The Book of The Sith :

"Now that We have achieved Victory, and The Jedi are no more, I shall institute 

A Rule of One

I have no intention of training my Successor..."

- Darth Sideous

Then your successor must, by necessity, be entirely 

Self-Taught...

Great Disturbers of The Force : When Three Men Enter, Two Men Leave

In The End — There can Be Only Two.

The Chymical Wedding

Always Two, There Are - 

No More, No Less.

A Master, and An Apprentice.

And an Applicant.

Which Shall Be Destroyed...?

The Master, or The Apprentice...?

Or The Applicant..?

Three Men Enter - Two Jedi, One Sith

Three Men Leave - 

One Jedi, One Sith, One Sith/Jedi

•WISH GRANTED•

Clearly, you can't.

Dooku never got this far —

He had to renounce The Jedi Order in order to claim his hereditary title of Count on his homeworld, and he never made it onto The Jedi Council -

So he was never a Jedi Master and a Sith Lord 

at the same time

Vader is A Jedi Master/Sith Lord who decides to serve Evil

(Because he thinks his wife and child are dead and there is nothing to live for)

Luke is a Sith Lord/Jedi Master who decides to be Good

(In order to redeem his Father)

This is Balance.

But then his father dies - 

This is a problem...