Duplicate fan speed in M106/7 IDEX Duplication Mode (#21208)

Marlin/src/gcode/temp/M106_M107.cpp

@@ -81,6 +81,9 @@ void GcodeSuite::M106() {
 
     // Set speed, with constraint
     thermalManager.set_fan_speed(pfan, speed);
+
+    if (TERN0(DUAL_X_CARRIAGE, idex_is_duplicating()))  // pfan == 0 when duplicating
+      thermalManager.set_fan_speed(1 - pfan, speed);
   }
 }
 
@@ -88,8 +91,13 @@ void GcodeSuite::M106() {
  * M107: Fan Off
  */
 void GcodeSuite::M107() {
-  const uint8_t p = parser.byteval('P', _ALT_P);
-  thermalManager.set_fan_speed(p, 0);
+  const uint8_t pfan = parser.byteval('P', _ALT_P);
+  if (pfan < _CNT_P) {
+    thermalManager.set_fan_speed(pfan, 0);
+
+    if (TERN0(DUAL_X_CARRIAGE, idex_is_duplicating()))  // pfan == 0 when duplicating
+      thermalManager.set_fan_speed(1 - pfan, 0);
+  }
 }
 
 #endif // HAS_FAN