The events of this fic take place shortly after Season 5 Episode 16 of Clone Wars, titled "Lawless". It is my explanation as to how two Sith so easily defeated can be a threat.
"Loading, initializing Holocron protocols. How may I help you?"
"Open information add protocol, authorization Palpatine-01"
"Processing, what information do you wish to add, Lord Palpatine?"
"A duel with Maul and his brother, and the reasons behind it."
"Yes Lord Palpatine, one moment... what question do you wish to be used to invoke this information?"
"File it under the Rule of Two in connection with the Balance of the Force."
"One moment... recording is active, you may speak when ready Lord Palpatine."
"Why was the Rule of Two instated? Because of the Balance of the Force. Observe the following events; a Sith survives a duel with a Jedi, but no one knows. Years later, he unites with his brother and begins to manipulate galactic events. Despite almost no training, and pathetic Force sensitivity, these two are an absolute and dangerous threat to the Sith. Why? Because of the Balance of the Force.
The Jedi of this time are like rabbits, continually growing in population and getting into everything. This makes them weaker than usual. The Force is a balance between Light and Dark, and for every new Jedi, the entirety of the Light side of the Force is divided by N+1. This is why the Rule of Two is so vital. Only a Master and an Apprentice, any more and the Dark Side is diluted. Because of this, all of the Light Side must be balanced in the Dark, and when the Dark is wielded by only two, those two Sith become unstoppable, capable of anything.
I forsee this working the other way. Should the Jedi not be completely annihilated, a single Jedi would balance two Sith, giving him power far beyond normal. A Jedi Apprentice, practically untrained, but being the last, could potentially defeat a Sith in a duel. In this case the Jedi must be dealt with purely with Force abilities. In a duel, the force tilts favorably into the balance of the Jedi, but with Force powers only, now the Jedi, while having the potential to defend himself from it, doesn't have the experience. Should a plan to eradicate the Jedi begin, this is the only way to deal with the single remaining straggler."
"Information complete, close file."
"File closed, Lord Palpatine, do you wish to add more at this time?"
"No, shut down."
"Yes, Lord Palpatine."