Is it at all possible that either Darth Tenebrous, Plaegueis, Sidious, or Tyranus could have secretly fully trained another apprentice by their deaths or by 32 BBY (whichever comes first)? If so, would there be any notable side effects from being trained in secrecy by each master? I admit that I have relatively sparse knowledge on most of these Sith lords during that time frame, but based on what knowledge I do have, Tenebrous seems to be the most likely due to our lack of information on him, so him training another apprentice before Plagueis could possibly be something that happened earlier in his life. The other three (as far as I know) seemed to be very busy during that time period and wouldn't have time to train a second apprentice. But I would like to get the opinion of the general community here since there are probably people who are more informed about these four Sith than I am, and I think that this is an interesting topic that could be fun to discuss.

5442826 Tyranus' last apprentice was Komari Vosa, who he killed after she fell to the dark side.

He then gave her dual sabers to Ventress.

5442829 Well that's interesting! I guess that rules out Tyranus, but thanks for the fact!

5442843 No problem, here's what she looks like: