There’s a discussion about sensors comparison on taulabs forum.
Basically, second sensors supplements the first one as it has different sampling frequency which allows to avoid some vibration issues and serves as a backup in case something happens with the first one.
All sensors are connected directly to the Raspberry Pi. Co-processor is used as a peripheral extension to the RPi to provide it with RC input and PWM outputs, no sensor sampling or flight calculation are done on it. Software and protocol are open-source and will be released soon.
Do you have any software design docs that show how each of the components are used on the new Navio2 board ? You speak of a co-processor. How does it work with APM ? Does APM have to be modified to take advantage of the Navio2 hardware redundancy like dual 9DOF and co-processors etc ?
Sensors are used pretty much the same way as on other autopilots. We have some explanation in Navio2 introduction post.
Co-processor is integrated into Linux sysfs by a kernel driver. APM works with /pwm and /rc-input interfaces provided by it. There are some modifications to the APM code regarding the co-processor and they will be pushed in main repository soon. Dual IMU support is already in APM.