Whether you use the plane or not is your decision. Both owners have the right to use it any time they want. By renting the plane, he is exposing you to liability for anything that goes wrong and kills a renter. Make sure he is paying for an insurance policy. He can sue you for contribution - your 50% share of the costs of the plane. You can sue him for disgorgement of profits from the rental of the plane.
Either start using the plane, sell it to him, or sue him to partition the plane by sale. In a partition by sale, the court will charge you for upkeep but credit you for rental fees. The profit from the plane sale will be split 50-50 and any setoffs will be applied to each person's share. You may end up owing money on the plane.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.