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, 0, 20, 0, 112) - map(distance2, 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`/`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">
****
As I understand, this is what you want:

    int motorPin = 9;
    int trigPin = 13;
    int echoPin = 12;
    int trigPin2 = 5;
    int echoPin2 = 4;
    int postdist = 112;
    int postdist2 = 112;
    
    void setup() {
      Serial.begin (9600);
      pinMode(trigPin, OUTPUT);
      pinMode(echoPin, INPUT);
      pinMode(trigPin2, OUTPUT);
      pinMode(echoPin2, INPUT);
      pinMode(motorPin, OUTPUT);
    }
    
    void loop() {
      //First Read
      int duration, distance;
      digitalWrite (trigPin, HIGH);
      delayMicroseconds (1000);
      digitalWrite (trigPin, LOW);
      duration = pulseIn (echoPin, HIGH);
      distance = (duration/2) / 29.1;
      int ltol = postdist - 5;
      int htol = postdist + 5;
      if (distance > 20 || !(distance > ltol && distance < htol)) {  // Distance from sensor
        Serial.println("Out of range");
        distance = postdist;
      }
      //Second Read
      int duration2, distance2;
      digitalWrite (trigPin2, HIGH);
      delayMicroseconds (1000);
      digitalWrite (trigPin2, LOW);
      duration2 = pulseIn (echoPin2, HIGH);
      distance2 = (duration2/2) / 29.1;
      int ltol2 = postdist2 - 5;
      int htol2 = postdist2 + 5;
      if (distance2 > 20 || !(distance2 > ltol2 && distance2 < htol2)) {  // Distance from sensor
        Serial.println("Out of range");
        distance2 = postdist2;
      }
      //Math:
      Serial.print("Distances-1: ");
      Serial.print(distance);
      Serial.print(" cm 2: ");
      Serial.print("Distances-1: ");
      Serial.print(distance2);
      Serial.println(" cm");
      postdist = distance;
      postdist2 = distance2;
      int speed = 112 + map(distance, 0, 20, 0, 112) - map(distance2, 0, 20, 0, 112); 
      if(speed < 60) {speed = 60;} //Modify 60 to meet your needs for minimum threshold
      analogWrite (motorPin, speed);
      Serial.print("Motor Speed: ");
      Serial.println(speed);
      delay(500);
    }

**Changelog:** forgot to edit `postdist`/`postdist2` at end of cycle, added minimum threshold, prevent very small changes in voltage to keep voltage stable and prevent noise.