Altolamprologus calvus are not overly aggressive cichlids, though male-to-male aggression is common. In my 75g tank I have three calvus - two males and a female. I purchased them together as juveniles when they were the same size and was thus unable to sex them at the time. Now that they're sub-adults, their genders are more apparent.
Typically, two males in a 75g is probably dicey. However, because cichlids have individual personalities, it is always possible that you can own docile specimens of a species not known to be docile.
Pictured above is my dominant male. Needless to say, I would consider him the tank boss. The dome-shaped structure that he's facing is a ceramic cichlid cave, which he calls home and serves as the center of his territory. You can just make out the entrance on the right side. This cave is up against the glass on the left-side of the tank (see full tank photo above). He doesn't venture very far from the cave. In fact, he never goes farther than the center of the tank and always remains to the left of the foreground plant in the full tank photo.
The sub-dominant male in the tank doesn't really have a territory, even though there is another identical cave near the center of the tank. He's pictured below and is almost a carbon copy of the dominant male. He's usually hovering around the rocks in the center. The two males don't pay much attention to each other and there is definitely little chasing. They just simply stay apart.
The lone female (pictured below) doesn't get much attention from either male. In fact, the dominant male is less tolerant of her than the sub-dominant male. She and the sub-dominant male co-exist pretty peacefully in the right half of the tank. She'll occasionally venture over to the left, but she usually stays on the back-side of the rock work where she's less visible to the dominant male. You can barely see her in the full tank photo - she's just to the left of the Hagen digital heater toward the rear of the tank. Notice her lighter color when compared to the two males.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub