I have neither of them though I’ve tried them via friend support. Chiron, for me, is better used for his buffs rather than his star gen. I don’t know how well he generates stars, but without a star gen buff, or a stars-per-turn buff (or 2030 CE) I doubt he can provide a consistent amount of stars for your DPS.
Not to mention that he has high natural star weight, so he draws in stars more than other classes (except for Riders). If his cards appear the same turn as the DPS Servant, it’s more likely that Chiron will take the bulk of the stars for himself, unless the other Servant is a Rider, or has a skill or CE to increase their star absorption.
Chiron is a great buffer but, to put him in a crit team, the third Servant will need to help provide stars in some way. Preferably by providing plenty of stars so that, even if Chiron takes some or most of the stars, there’s still plenty leftover for the main DPS to use.
Sigurd has a one-turn star gen buff he can use on Chiron, or whoever the third Servant will be, but he lacks ways to overcome Chiron’s star weight, so expect him to lose stars to Chiron if their cards appear on the same turn.
Also, the two of them together may not necessarily work very well. Sigurd will likely come out for Lancer bosses given that he is an ST DPS. Those Lancer bosses will murder Chiron easily due to class advantage.
You could probably use them for Zerk enemies, or large dragons (usually Riders so bringing Caster support is difficult) but Chiron may have to sit out Lancer fights.