MK2_MULTIPLEXER does not require E2/E3 stepper pins

Marlin/SanityCheck.h

@@ -1056,24 +1056,25 @@ static_assert(1 >= 0
 /**
  * Test Extruder Stepper Pins
  */
-#if E_STEPPERS > 4
+#if DISABLED(MK2_MULTIPLEXER) // MK2_MULTIPLEXER uses E0 stepper only
+  #if E_STEPPERS > 4
     #if !PIN_EXISTS(E4_STEP) || !PIN_EXISTS(E4_DIR) || !PIN_EXISTS(E4_ENABLE)
       #error "E4_STEP_PIN, E4_DIR_PIN, or E4_ENABLE_PIN not defined for this board."
     #endif
-#elif E_STEPPERS > 3
+  #elif E_STEPPERS > 3
     #if !PIN_EXISTS(E3_STEP) || !PIN_EXISTS(E3_DIR) || !PIN_EXISTS(E3_ENABLE)
       #error "E3_STEP_PIN, E3_DIR_PIN, or E3_ENABLE_PIN not defined for this board."
     #endif
-#elif E_STEPPERS > 2
+  #elif E_STEPPERS > 2
     #if !PIN_EXISTS(E2_STEP) || !PIN_EXISTS(E2_DIR) || !PIN_EXISTS(E2_ENABLE)
       #error "E2_STEP_PIN, E2_DIR_PIN, or E2_ENABLE_PIN not defined for this board."
     #endif
-#elif E_STEPPERS > 1
+  #elif E_STEPPERS > 1
     #if !PIN_EXISTS(E1_STEP) || !PIN_EXISTS(E1_DIR) || !PIN_EXISTS(E1_ENABLE)
       #error "E1_STEP_PIN, E1_DIR_PIN, or E1_ENABLE_PIN not defined for this board."
     #endif
+  #endif
 #endif
-
 /**
  * Endstop Tests
  */