Registered: Aug 2016
Location: Pennsylvania, United States
Maybe because Vader was seen as capable of killing Sidious by Sidious himself? Vader has the feats to back that assumption,despite Sidious being overly paranoid. Two Vader's are more than enough to kill just about anybody, save maybe GM Luke. Anybody that Sidious thinks could kill him is not to be underestimated, and two of them is just crazy.
Registered: Jun 2016
Location: The Throne of the Sheevites
This?
Yes, Vader remained strong in the Force, but strong enough to succeed me? Never. Granted, he was strong enough to kill me. But that only lasted for so long.
-Jedi vs. Sith: The Essential Guide To The Force
It refers to Palpatine's death in RotJ, which was circumstantial. This is an invalid piece of evidence. Vader's feats pale in comarison to the Emperor's and Yoda's. He isn't comparable to them. Two Vaders isn't going to make any difference.
I'd say his statement align with what's implied by other characters, stated by Vader, implied by the narrative, and even Lucas' Vader being 8/10 Palpatine's power quote.
It's a running theme in Star Wars that two Vader level characters --- be it Vader/Luke, Vader/Starkiller, Vader/Murr, etc can pose a legit threat to Sidious.
Yoda loses because he hasn't shown the feats, to defeat two Vader level being simultaneously.