It's a common scenario in criminal defense: two or more people are charged related to the same crime or group of crimes. When you aren't the only defendant, you have to consider what the other person might do or say in your defense strategy. It's also probably not in your best interest to share an attorney or strategy with another person.
When two or more people are charged with a crime, there are several common ways that the other defendants will approach the case. First, they might deny all wrongdoing on either their part or your part. This is known as a complete denial, and it might also include an alibi for the other person. If the person was not in town, for example, they couldn't have been involved in the crime. A good scenario for you would be where the other person's alibi and denial help prove yours. If you were both out of town together, then neither of you could have committed the crimes.
Another scenario involves a partial denial. The other person might admit to certain crimes but not others. This could be part of a plea bargain that may or may not impact you. It depends on what the bargain is. The other person might agree to give testimony about your involvement in a crime so they can plead to a lower charge.
The other person might deny the crime altogether and attempt to pin it on you or someone else. They might also admit to the crime, including you in their admission.
No matter how the scenario plays out, if another defendant is involved, it is important to understand that what they do or say can impact your own defense. Working with an attorney who has your best interests in mind can help you protect your own rights
Source: Findlaw, "Criminal Defense Strategies," accessed Oct. 02, 2015