Do you mean another of the same serra or like two different serra species living together. Its risky business either way. Geryi and Golds are the more common serras that are attempted to cohab. I wouldnt recommend trying to cohab serras unless you have good experience and a lot of time to monitor their activity.
The most serra know is cohad and successful are Geryi and Gold Spilo/Mac together. But really recommend with a good experience keeper only, and it's a risk.
I had 9 gold spilo together in my 300gallon from 2" to 8" with successful before i sold them. I know a guy who breed these mac before.
Even if you were able to cohab say a serra and a pygo for a little bit I think it would always end with a dead fish. There are just too many factors workin against it like growth rate, territory and just overall aggression.
We'll see before much longer what happens I guess-
There is a guy I know that is from turkey that is cohabing a mix pygo tank with one sanchezi-Things are going good so far---
If you dont mind I'd like to know a little bit more about your cohab. What size are all of them, how long have they been together and what do you think is the main reason they live together peacefully.