I'm going to play the devil's advocate position here and say that I would not be so hasty to say that you couldn't keep two Mbu together. Mbu have a whole different energy level than Fahaka (or at least the Fahaka I have seen...). For example, Fahaka seem to be fish killers whereas Mbu are not.
I'm not about to run out there and buy another Mbu just to see what happens but I would definitely not make a blanket statement like "it won't work". The guy, above, appears to not be having any issues, at least for the time being. We "assume" a lot of things about Mbu, based on what Fahaka do but the reality is that there are not many of us who have had adult Mbu for long periods of time and even less, who tried something like this. So, I'm not saying it would work but if we are just looking at the evidence, there is moe to suggest it could than it couldn't.
Just my 2-cents.