I have not heard that either. Where did you read that?
Just out of boredom I was messing about with the community creator on "think fish"... Which is crap by the way. Their "guide line stocking levels" are ridiculous. If I were to stock my tank with the amount that they say I could there would be more fish in there than water! Plus it doesn't take into consideration tank size.
Anyway, You choose Severum and it instantly adds two, so I deleted one, and then at the bottom of the page where it states any issues it said "Severums must be kept in twos or more".