Even though two defendants are charged with committing the same offense at the same time as co-conspirators or parties to an offense, they are not required to be made or to accept the same plea offer.  Many times, prosecutors will offer the same deal to both.  However, if one defendant has a substantially worse record than the other, they may offer that defendant a stiffer sentence.  If you want to take a deal that is right for you, then do so.  Each person has to evaluate their plea offer in light of what will be best for them in the long run.... even if it's different than what their co-defendant had in mind.