How can I scan for a HIGH signal on digitalRead during the main loop, while running other things?
Simplify, and show your setup code.

First, do you have a pinMode call in your setup function:

void setup() {
  pinMode(A15, INPUT_PULLUP);
  Serial.begin(115200);
}


Then use test code like this:

void loop() {
   if (digitalRead(A15) == HIGH) {
      Serial.println("Pin A15 is HIGH");
   } else {
      Serial.println("Pin A15 is LOW");
   }
}


Connect your A15 pin directly to ground with a jumper and run the sketch above. Open the serial monitor and see what is output. It should be LOW. Then disconnect the jumper and check again. It should now be HIGH. Finally, connect the jumper between A15 and +5V and check the output again. It should still show HIGH.

That lets you troubleshoot just the software piece. Once you get that working, connect pin A15 to the remote Arduino's output pin and try again.