Yes, they like to be with other fronts, but you can keep them individually with other fish, as many do. Fronts are among the more individualistic fish in terms of temperament ime with them (dozen years or more) and some will be much happier with other fronts and others won't make as much difference. But from what you posted and what I see in your tank I'd suspect the nitrite first and other fish being more aggressive species than your front second.
Also, I wouldn't keep just two fronts together. They're not a fish to keep in pairs in most circumstances. You can keep one or several, but just two of them are not likely to work very well.