Add USB serial support to SERIAL_PORT_2 on DUE (#17245)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/HAL/DUE/MarlinSerialUSB.cpp

@@ -29,7 +29,7 @@
 
 #include "../../inc/MarlinConfig.h"
 
-#if SERIAL_PORT == -1
+#if HAS_USB_SERIAL
 
 #include "MarlinSerialUSB.h"
 
@@ -283,8 +283,12 @@ void MarlinSerialUSB::printFloat(double number, uint8_t digits) {
 }
 
 // Preinstantiate
-MarlinSerialUSB customizedSerial1;
-
-#endif // SERIAL_PORT == -1
+#if SERIAL_PORT == -1
+  MarlinSerialUSB customizedSerial1;
+#endif
+#if SERIAL_PORT_2 == -1
+  MarlinSerialUSB customizedSerial2;
+#endif
 
+#endif // HAS_USB_SERIAL
 #endif // ARDUINO_ARCH_SAM