This is *very* simple using the **[`map()` instruction](http://arduino.cc/en/reference/map).** Here's the code you would use (feel free to modify values:

    int speed = 112 + map(distance_1, 0, 20, 0, 112) - map(distance_1, 0, 20, 0, 112);

So if both sensors gave equal values, they would offset each other. To make it still work if an object is removed, you could just use the last `distance_1`/`distance_2` value and update the other.
****
One issue with this approach is, if the line of transformation of the object is *not* perpendicular to the line of the pings, the value will go:
> 10 → 11 → 12 → 13 → 14 → 15 → 16 → 17 → 18 → 19 → 20 → [Out of range]

**I've added some pretty animations to show you what I mean:**

Perpendicular:

<img src="https://i.sstatic.net/jmOp6.gif" width="200" height="200">

Collinear:

<img src="https://i.sstatic.net/tnk9M.gif" width="200" height="200">



[Will add code in a bit]