Using HTC Duo cameras to calculate distance
have you tried any of the measuring apps on play, They might have a dual camera support..

Most of them calculate your height and the distance it takes to reach the ground using the gyroscope

but i haven't heard of one for dual cameras.. It means you would have to manually configure the distance between cameras on every individual device .. It would be a nice app to see in development but it'd be a very small customer base to make an application around, considering the device compatibility wouldn't be very extensive !