In terms of sabers, I can see Sidious winning. Even by TPM Sidious comes of as the better duelist before his supreme feats happened; I've debated that Plagueis would beat Palpatine in sabers because of their relationship and setup pretty decisively as of TPM (I can provide quotes of the analysis), but that was before Palpatine went on to live for many decades more, so that wouldn't be a case here. Tenebrous won't be enough to tip the scales here; you can vaguely place him a very wide area of probable skill, but that just will not be enough.
As far as the Force goes, here Tenebrous is a lot more legitimate threat, and Plagueis himself is closer to Palpatine in the Force than he is in sabers. I can see team winning here.
All out - probably goes just like sabers.