Don't hide anything. Go and tell company B that you have joined company A, but you always wanted to join B. It would work out well. But be ready with satishfying answers for the uncomfortable questions. Questions like "what if company C gives you an offer, would you quit B?", "why did you join A, when you always wanted B?" etc will be asked. May be it's a personal matter, but I believe one shouldn't join a company just because it's a MNC or it's CMM level five. Go by what you get out of this deal.
