Fix sanity checks for dual axes (#10633)

.travis.yml

   #      PROBE_MANUALLY feature, with LCD support,
   #      ULTIMAKERCONTROLLER, FILAMENT_LCD_DISPLAY, FILAMENT_WIDTH_SENSOR,
   #      PRINTCOUNTER, NOZZLE_PARK_FEATURE, NOZZLE_CLEAN_FEATURE, PCA9632,
-  #      Z_DUAL_STEPPER_DRIVERS, Z_DUAL_ENDSTOPS, BEZIER_CURVE_SUPPORT, EXPERIMENTAL_I2CBUS,
+  #      Z_DUAL_ENDSTOPS, BEZIER_CURVE_SUPPORT, EXPERIMENTAL_I2CBUS,
   #      ADVANCED_PAUSE_FEATURE, ADVANCED_PAUSE_CONTINUOUS_PURGE, PARK_HEAD_ON_PAUSE, LCD_INFO_MENU,
   #      EEPROM_SETTINGS, EEPROM_CHITCHAT, M100_FREE_MEMORY_WATCHER,
   #      INCH_MODE_SUPPORT, TEMPERATURE_UNITS_SUPPORT
   - opt_enable PROBE_MANUALLY AUTO_BED_LEVELING_BILINEAR G26_MESH_EDITING LCD_BED_LEVELING ULTIMAKERCONTROLLER
   - opt_enable EEPROM_SETTINGS EEPROM_CHITCHAT M100_FREE_MEMORY_WATCHER M100_FREE_MEMORY_DUMPER M100_FREE_MEMORY_CORRUPTOR INCH_MODE_SUPPORT TEMPERATURE_UNITS_SUPPORT
   - opt_enable ULTIMAKERCONTROLLER SDSUPPORT
-  - opt_enable PRINTCOUNTER NOZZLE_PARK_FEATURE NOZZLE_CLEAN_FEATURE PCA9632 USE_XMAX_PLUG
+  - opt_enable PRINTCOUNTER NOZZLE_PARK_FEATURE NOZZLE_CLEAN_FEATURE PCA9632
   - opt_enable_adv BEZIER_CURVE_SUPPORT EXPERIMENTAL_I2CBUS
   - opt_enable_adv ADVANCED_PAUSE_FEATURE ADVANCED_PAUSE_CONTINUOUS_PURGE FILAMENT_LOAD_UNLOAD_GCODES PARK_HEAD_ON_PAUSE LCD_INFO_MENU M114_DETAIL
   - opt_set_adv PWM_MOTOR_CURRENT {1300,1300,1250}
   - opt_enable_adv Z_DUAL_STEPPER_DRIVERS Z_DUAL_ENDSTOPS
   - pins_set RAMPS X_MAX_PIN -1
   - opt_add_adv Z2_MAX_PIN 2
+  - opt_enable USE_XMAX_PLUG
   - build_marlin_pio ${TRAVIS_BUILD_DIR} ${TEST_PLATFORM}
 
   #############################

Marlin/src/inc/SanityCheck.h

 #if ENABLED(X_DUAL_ENDSTOPS)
   #if !X2_USE_ENDSTOP
     #error "You must set X2_USE_ENDSTOP with X_DUAL_ENDSTOPS."
-  #elif X2_USE_ENDSTOP == _X_MIN_ && DISABLED(USE_XMIN_PLUG)
-    #error "USE_XMIN_PLUG is required when X2_USE_ENDSTOP is _X_MIN_."
-  #elif X2_USE_ENDSTOP == _X_MAX_ && DISABLED(USE_XMAX_PLUG)
-    #error "USE_XMAX_PLUG is required when X2_USE_ENDSTOP is _X_MAX_."
-  #elif X2_USE_ENDSTOP == _Y_MIN_ && DISABLED(USE_YMIN_PLUG)
-    #error "USE_YMIN_PLUG is required when X2_USE_ENDSTOP is _Y_MIN_."
-  #elif X2_USE_ENDSTOP == _Y_MAX_ && DISABLED(USE_YMAX_PLUG)
-    #error "USE_YMAX_PLUG is required when X2_USE_ENDSTOP is _Y_MAX_."
-  #elif X2_USE_ENDSTOP == _Z_MIN_ && DISABLED(USE_ZMIN_PLUG)
-    #error "USE_ZMIN_PLUG is required when X2_USE_ENDSTOP is _Z_MIN_."
-  #elif X2_USE_ENDSTOP == _Z_MAX_ && DISABLED(USE_ZMAX_PLUG)
-    #error "USE_ZMAX_PLUG is required when X2_USE_ENDSTOP is _Z_MAX_."
+  #elif X2_USE_ENDSTOP == _XMIN_ && DISABLED(USE_XMIN_PLUG)
+    #error "USE_XMIN_PLUG is required when X2_USE_ENDSTOP is _XMIN_."
+  #elif X2_USE_ENDSTOP == _XMAX_ && DISABLED(USE_XMAX_PLUG)
+    #error "USE_XMAX_PLUG is required when X2_USE_ENDSTOP is _XMAX_."
+  #elif X2_USE_ENDSTOP == _YMIN_ && DISABLED(USE_YMIN_PLUG)
+    #error "USE_YMIN_PLUG is required when X2_USE_ENDSTOP is _YMIN_."
+  #elif X2_USE_ENDSTOP == _YMAX_ && DISABLED(USE_YMAX_PLUG)
+    #error "USE_YMAX_PLUG is required when X2_USE_ENDSTOP is _YMAX_."
+  #elif X2_USE_ENDSTOP == _ZMIN_ && DISABLED(USE_ZMIN_PLUG)
+    #error "USE_ZMIN_PLUG is required when X2_USE_ENDSTOP is _ZMIN_."
+  #elif X2_USE_ENDSTOP == _ZMAX_ && DISABLED(USE_ZMAX_PLUG)
+    #error "USE_ZMAX_PLUG is required when X2_USE_ENDSTOP is _ZMAX_."
   #elif !HAS_X2_MIN && !HAS_X2_MAX
     #error "X2_USE_ENDSTOP has been assigned to a nonexistent endstop!"
   #elif ENABLED(DELTA)
 #if ENABLED(Y_DUAL_ENDSTOPS)
   #if !Y2_USE_ENDSTOP
     #error "You must set Y2_USE_ENDSTOP with Y_DUAL_ENDSTOPS."
-  #elif Y2_USE_ENDSTOP == _X_MIN_ && DISABLED(USE_XMIN_PLUG)
-    #error "USE_XMIN_PLUG is required when Y2_USE_ENDSTOP is _X_MIN_."
-  #elif Y2_USE_ENDSTOP == _X_MAX_ && DISABLED(USE_XMAX_PLUG)
-    #error "USE_XMAX_PLUG is required when Y2_USE_ENDSTOP is _X_MAX_."
-  #elif Y2_USE_ENDSTOP == _Y_MIN_ && DISABLED(USE_YMIN_PLUG)
-    #error "USE_YMIN_PLUG is required when Y2_USE_ENDSTOP is _Y_MIN_."
-  #elif Y2_USE_ENDSTOP == _Y_MAX_ && DISABLED(USE_YMAX_PLUG)
-    #error "USE_YMAX_PLUG is required when Y2_USE_ENDSTOP is _Y_MAX_."
-  #elif Y2_USE_ENDSTOP == _Z_MIN_ && DISABLED(USE_ZMIN_PLUG)
-    #error "USE_ZMIN_PLUG is required when Y2_USE_ENDSTOP is _Z_MIN_."
-  #elif Y2_USE_ENDSTOP == _Z_MAX_ && DISABLED(USE_ZMAX_PLUG)
-    #error "USE_ZMAX_PLUG is required when Y2_USE_ENDSTOP is _Z_MAX_."
+  #elif Y2_USE_ENDSTOP == _XMIN_ && DISABLED(USE_XMIN_PLUG)
+    #error "USE_XMIN_PLUG is required when Y2_USE_ENDSTOP is _XMIN_."
+  #elif Y2_USE_ENDSTOP == _XMAX_ && DISABLED(USE_XMAX_PLUG)
+    #error "USE_XMAX_PLUG is required when Y2_USE_ENDSTOP is _XMAX_."
+  #elif Y2_USE_ENDSTOP == _YMIN_ && DISABLED(USE_YMIN_PLUG)
+    #error "USE_YMIN_PLUG is required when Y2_USE_ENDSTOP is _YMIN_."
+  #elif Y2_USE_ENDSTOP == _YMAX_ && DISABLED(USE_YMAX_PLUG)
+    #error "USE_YMAX_PLUG is required when Y2_USE_ENDSTOP is _YMAX_."
+  #elif Y2_USE_ENDSTOP == _ZMIN_ && DISABLED(USE_ZMIN_PLUG)
+    #error "USE_ZMIN_PLUG is required when Y2_USE_ENDSTOP is _ZMIN_."
+  #elif Y2_USE_ENDSTOP == _ZMAX_ && DISABLED(USE_ZMAX_PLUG)
+    #error "USE_ZMAX_PLUG is required when Y2_USE_ENDSTOP is _ZMAX_."
   #elif !HAS_Y2_MIN && !HAS_Y2_MAX
     #error "Y2_USE_ENDSTOP has been assigned to a nonexistent endstop!"
   #elif ENABLED(DELTA)
 #if ENABLED(Z_DUAL_ENDSTOPS)
   #if !Z2_USE_ENDSTOP
     #error "You must set Z2_USE_ENDSTOP with Z_DUAL_ENDSTOPS."
-  #elif Z2_USE_ENDSTOP == _X_MIN_ && DISABLED(USE_XMIN_PLUG)
-    #error "USE_XMIN_PLUG is required when Z2_USE_ENDSTOP is _X_MIN_."
-  #elif Z2_USE_ENDSTOP == _X_MAX_ && DISABLED(USE_XMAX_PLUG)
-    #error "USE_XMAX_PLUG is required when Z2_USE_ENDSTOP is _X_MAX_."
-  #elif Z2_USE_ENDSTOP == _Y_MIN_ && DISABLED(USE_YMIN_PLUG)
-    #error "USE_YMIN_PLUG is required when Z2_USE_ENDSTOP is _Y_MIN_."
-  #elif Z2_USE_ENDSTOP == _Y_MAX_ && DISABLED(USE_YMAX_PLUG)
-    #error "USE_YMAX_PLUG is required when Z2_USE_ENDSTOP is _Y_MAX_."
-  #elif Z2_USE_ENDSTOP == _Z_MIN_ && DISABLED(USE_ZMIN_PLUG)
-    #error "USE_ZMIN_PLUG is required when Z2_USE_ENDSTOP is _Z_MIN_."
-  #elif Z2_USE_ENDSTOP == _Z_MAX_ && DISABLED(USE_ZMAX_PLUG)
-    #error "USE_ZMAX_PLUG is required when Z2_USE_ENDSTOP is _Z_MAX_."
+  #elif Z2_USE_ENDSTOP == _XMIN_ && DISABLED(USE_XMIN_PLUG)
+    #error "USE_XMIN_PLUG is required when Z2_USE_ENDSTOP is _XMIN_."
+  #elif Z2_USE_ENDSTOP == _XMAX_ && DISABLED(USE_XMAX_PLUG)
+    #error "USE_XMAX_PLUG is required when Z2_USE_ENDSTOP is _XMAX_."
+  #elif Z2_USE_ENDSTOP == _YMIN_ && DISABLED(USE_YMIN_PLUG)
+    #error "USE_YMIN_PLUG is required when Z2_USE_ENDSTOP is _YMIN_."
+  #elif Z2_USE_ENDSTOP == _YMAX_ && DISABLED(USE_YMAX_PLUG)
+    #error "USE_YMAX_PLUG is required when Z2_USE_ENDSTOP is _YMAX_."
+  #elif Z2_USE_ENDSTOP == _ZMIN_ && DISABLED(USE_ZMIN_PLUG)
+    #error "USE_ZMIN_PLUG is required when Z2_USE_ENDSTOP is _ZMIN_."
+  #elif Z2_USE_ENDSTOP == _ZMAX_ && DISABLED(USE_ZMAX_PLUG)
+    #error "USE_ZMAX_PLUG is required when Z2_USE_ENDSTOP is _ZMAX_."
   #elif !HAS_Z2_MIN && !HAS_Z2_MAX
     #error "Z2_USE_ENDSTOP has been assigned to a nonexistent endstop!"
   #elif ENABLED(DELTA)