The rule of two nowhere says there are not hidden Sith splinter groups according to the eu. Simply that its Bane's line of successors.

Just as why cannot there be Jedi groups not governed or answerable to the council?

We are lead to believe there were limited Jedi and Palpatine somehow destroyed them all with Vader's help.

But in the Eu there are some groups who use the force who neither claim to be jedi or sith.

In reality in think there would be far more diversity and different denominations among Jedi and Sith as there are in and outside of orthodox Christianity.

Using that reference because i am not well versed in if there are schisms or different tribes of belief in other religions.