It means:

The french Comediéns did as much (as someone else did?) on their end, and the trial …

(I would keep the french word Comédiens maybe to underscore that these are actors of the Comédie Française, most probably)

I analyze “Autant en firent de leur côté les Comédiens français” to mean: “_les Comédiens français en firent autant (que quelqu’un d’autre) de leur côté…”, and the prior context should make clear what that en refers to.

Can you show us the sentence immediately before this one?

(NB: the OP added the sentence immediately before in his post following this question).

The sentence seems to be old and dusty (because of an inversion of words that feel unnatural). Nowaday you would more likely say something like :

[De leur côté,] les Comédiens français en firent autant …

“En faire autant” means to do the same thing.