To answer your question I doubt you could ad a second snakehead after the fact. once a snakehead is kept solo, even if only for a short time ,it is VERY unlikely to ever except a tankmate/mate. So start them together very young if your going to try.
Also it's worth mentioning the group behaviour is a juvenile thing. They are not schooling/pack hunting fish like Pleuros . As juvies they'll school and be friendly to their own kind but once they mature they pair (or don't) and become highly territorial toward their own kind.
Hope that helps.
I was thinking the same. The MFKer Inter has 4 Platinum Argus. He must have kept them together since little.
Here is a video of Argus in a group. Interesting communication between them. Must have been together since small and the space should help lessen aggression too.
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But I agree with you. It is highly risky especially once it has been kept solitary. And if disaster occurs housing the two separately becomes a problem unless there's an extra tank sitting at home.
Thank you for the kind reply.