Making a Y-wire to connect the trigger- and the echo-pins to one of the digital pins - pin 10 for example. And at the start of your code assign the pin# to both symbols..

const unsigned int PIN_UStrigg = 10; 

const unsigned int PIN_USecho = 10;

I don't use the HC-SR04 library - I wrote my own code to handle it, to keep it small in flash and short in execution time. And my opinion, there's no need to deal with floats or long integers. 

If an obstacle is more then 50cm away, there is still enough place to walk forward..

//--------------------------------------------------------------- ULTRA SOUND SENSOR 
//
// The ultrasound sensor has a trigger pin to start the measuring and an echo pin
// that's high as long the sensor waits for the echo. The two pins of the HC_SR04
// can be connected together, so that only one Arduino pin is needed for triggering
// and getting the pulseIn() result (PIN_Trigger == PIN_Echo).
// To save runtime the max distance that will be returned is limited to 86cm. 
//
byte USgetDistance() {

// 65535us = 112,99m = far far away
word dist = 0;
// Trigger
pinMode(PIN_UStrigg,OUTPUT);
digitalWrite(PIN_UStrigg, HIGH); // Trigger Impuls 10 us
delayMicroseconds(10); // Default 10
digitalWrite(PIN_UStrigg, LOW);
// Pingen
pinMode(PIN_USecho,INPUT);
// Echo Timeout = 5000us = max. dist. 86cm
// to get up to 255cm range set timeout <= 14790us
dist = pulseIn(PIN_USecho, HIGH, 5000); // Echo-Zeit messen
// if US distance sensor's echo and trigger pins are connected
// inhibit retriggering by echo pulse
pinMode(PIN_UStrigg,OUTPUT);
digitalWrite(PIN_UStrigg, LOW);
// pulseIn() returns 0, if echo time > timeout 
if (!dist) dist = 5000; // no echo - assume max distance
dist = (dist+29) / 29 / 2; // us to cm distance
// Serial.print(dist);
// Serial.println("cm");
return (byte)dist;
} // USgetDistance()


void USshowDistance() {
byte dist = USgetDistance();
// for displaying a 'bargraph'..
byte level = map(dist, 0, 80, 0, 40);
Serial.print(F("Distance "));
while (level--) {
Serial.write('#');
}
Serial.write(' ');
// ..and the measured distance
Serial.println(dist, DEC);
} // USshowDistance()
//

I got this idea and tested it, because there also 3-pin US-Sensors available and also supported by the HC-SR04 or US-Sensors libraries.