tpasaro5:
Time will start when the transmission from arduino 1 will trigger. Arduino 2 will receive it and immediately send a transmission back to arduino 1. Once arduino 1 receives the transmission by arduino 2 the time will stop.That I have figured out already.
I would love to hear how you plan to accomplish this if Arduino 1 and 2 are not connected by wires or radios. Do tell, please.
For the future...some great reading on the HC-SR04:
HC-SR04 | David Pilling Potentially useful information for the OP and others with similar aspirations starts at "Thinking that the comparator with threshold input was hiding what was actually going on..." Could allow an Arduino to monitor an HC-SR04 and identify when the sensor has received an ultrasonic pulse (without needing to trigger it and wait for an echo or incoming pulse from an unconnected transmitter).
https://uglyduck.ath.cx/ep/archive/2014/01/Making_a_better_HC_SR04_Echo_Locator.html Schematic of HC-SR04 and explanation. Beware: your HC-SR04 may be somewhat different.