I don't think currently you can have this setup out of the box with ArduPilot. They way I would try to go about it is have the two sensors looking up and down on i2c, unfortunately currently you can only specify a single PRX_TYPE parameter (meaning you can have either i2c or serial at a time).
When I was testing i2c sensors for obstacle avoidance back in November i run into some problems: https://discuss.ardupilot.org/t/lidar-doesnt-work-when-rngfnd-orient-set-to-up-24/23122/4, assuming they were resolved there are two ways I would go around this:
- Modify the firmware to add a new proximity type (TeraRanger Tower + i2c) (or to make it even more clean create PRX_TYPE_2)
- Add a companion computer that can collect data from all your sensor and send them as mavlink messages (PRX_TYPE MAVLink) (I never used proximity this way so I can't tell how reliable it is)
In both cases you would quite likely need to modify the ArduCopter firmware.
Hope any of that helps!