"You should decline the offer as soon as you have accepted another offer or made a definitive decision not to accept."

"In rare cases, your final decision (either verbal or written) may trigger a decision to negotiate with you. In case you are given the opportunity to obtain what you want from the offer, you should be prepared to conduct the negotiation."

I understand that this is rare, but I wonder if you've really thought the scenario through. You've already accepted job offer A, but now you're negotiating job offer B? How would you explain your behavior to the first company if you end up taking the second offer?