MK2_MULTIPLEXER does not require E2/E3 stepper pins

Marlin/src/inc/SanityCheck.h

@@ -1076,6 +1076,7 @@ static_assert(1 >= 0
 /**
  * Test Extruder Stepper Pins
  */
+#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."
@@ -1093,7 +1094,7 @@ static_assert(1 >= 0
       #error "E1_STEP_PIN, E1_DIR_PIN, or E1_ENABLE_PIN not defined for this board."
     #endif
   #endif
-
+#endif
 /**
  * Endstop Tests
  */