first thought I have is tire pressure. But I need some clarification; what do you mean by the "insides of both tires" Are you talking about the center of the tread area, or are you talking about the outer edge of the tire between the tread and sidewall on the backside near the hub?
If you are talking about the center of the tread is wearing, I would suspect your tire pressure is to high and you are pushing the tread out so it is wearing uneven.