Well, it depends on what you mean for "center". Bruxelles itself is quite small, and the urban city is composed of the total of the 19 municipalities. Living in real Bruxelles, the very center can be substantially less expensive than most of the other zones. I live in one of the other municipalities and I'm still no more than 10/15' walking from the center. To live in a good zone (Ixelles, Etterbeek, the European Quarter...) you will need I guess 3k monthly with both salaries (an average rent is ~800/1000€