This paper presents a study on the infield self-calibration of two rigidly connected IMUs' intrinsic parameters, without the aid of any external sensors, equipment, or specialized procedures. Specifically, we consider the calibration of gyroscope biases, gyroscope scale factors, and accelerometer biases, using only IMU data and known extrinsics between the two IMUs. We focus on the observability analysis of this system, and show that all gyroscope intrinsic parameters and a portion of accelerometer biases are observable, with information from both IMUs and sufficient motion. Moreover, we identify the additional unobservable directions in the intrinsic parameters that arise from various degenerate motions. Finally, we validate our observability findings through numerical simulations, and assess our system's calibration accuracy using real-world data.