Improve altitude control

Hello. I need your help. My drone flies over corn fields at a speed of 8-10 m / s, and I would like to improve height control with lidar / sonar. Can I connect two lidars / sonars in a downward direction? How will the pixhawk height be considered (one priority or average), or can it be done using arduino (get both values and send the average to the FC). Many thanks.
P.S. what is the best firmware for these purposes 3.6 or 4.0