Generally speaking If a person is not being interrogated then Miranda right not required. Contact a local criminal attorney to investigate the facts of your arrest & to assist you with your case.
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
Under the law of parties you can both be charged for the full amount of the items you both stole because this was one criminal enterprise. It really doesn't matter that you weren't Mirandized. This only becomes relevant if the state tries to use statements you made against you at trial if those statements were made after you were taken into custody.
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The State may charge you for the total amount that was taken. They may also decide to charge you for what you had and charge your friend for what they had. Miranda usually means little unless you answered questions asked of you by the police. I am not sure what your question was, but it appears the state, thus far, has not over-reached its authority. An attorney would have access to the case and would be in a better position to find out what you are really charged with.