If you feel second compay is better than first one you can post pone your joining date in first company. And explain the situation with second company that you are interested in their offer but you already have offer and so if they can take decision as early as possible it will be helpful for you.
In other way just join in first company, and when you got the second offer if you feel it better than first one just resign and join in second company. But dont just escape from first one with out saying anything. Thats bad that I seen lot of people leaving the company without saying single
word when they got another offer.