I found when keeping NW cichlids the more the better. 2 was NEVER good an oscar and a severum alone in a tank could be war. You never know but I would not do it unless you have a 100Gal and other south american cichlids to level the aggression out. Each cichlid has it's own personality so if you know your oscar is a bully I think leave him alone.
Severums are my Favorite. The only reason I stopped keeping them is because I like planted tank. I miss mine. Someday I will get one again. Anyways depending on your Severums temperament a blue acara would work they tend to stay low in the tanks and Severums are mid level swimmer. I kept my Severum with a blue acara years ago no problems. I have an electric blue acara now with Discus and it is fine they keep to them self.