I would assume you have either two males, or a male/female pair that will not bond. In both cases, this will not improve, and the bullied fish will be dead before long if they are kept together. Once this stage has been reached, the tank size and aquascape won't usually matter much.
You use the term pair, but I don't know if you actually mean male/female, or if you just mean two fish left. But I'll explain the male/female problem. This male of Mikrogeophagus ramirezi, and the male of the closely-related species M. altispinosus, must select their own mate. The pair then bond, normally for life. You can initially either buy several fish and allow them to form their own bonded pair(s) [but then you have other fish to deal with somehow], or carefully observe them in the store tank and if any have bonded it will be obvious by their interactive behaviours.