🚸 Strict index 2 for M913 / M914 XY (#24680)

Marlin/src/gcode/feature/trinamic/M911-M914.cpp

         #if X_HAS_STEALTHCHOP || X2_HAS_STEALTHCHOP
           case X_AXIS:
             TERN_(X_HAS_STEALTHCHOP,  if (index < 2) TMC_SET_PWMTHRS(X,X));
-            TERN_(X2_HAS_STEALTHCHOP, if (!(index & 1)) TMC_SET_PWMTHRS(X,X2));
+            TERN_(X2_HAS_STEALTHCHOP, if (!index || index == 2) TMC_SET_PWMTHRS(X,X2));
             break;
         #endif
 
         #if Y_HAS_STEALTHCHOP || Y2_HAS_STEALTHCHOP
           case Y_AXIS:
             TERN_(Y_HAS_STEALTHCHOP,  if (index < 2) TMC_SET_PWMTHRS(Y,Y));
-            TERN_(Y2_HAS_STEALTHCHOP, if (!(index & 1)) TMC_SET_PWMTHRS(Y,Y2));
+            TERN_(Y2_HAS_STEALTHCHOP, if (!index || index == 2) TMC_SET_PWMTHRS(Y,Y2));
             break;
         #endif
 
    * M914: Set StallGuard sensitivity.
    */
   void GcodeSuite::M914() {
-
     bool report = true;
     const uint8_t index = parser.byteval('I');
     LOOP_NUM_AXES(i) if (parser.seen(AXIS_CHAR(i))) {
         #if X_SENSORLESS
           case X_AXIS:
             if (index < 2) stepperX.homing_threshold(value);
-            TERN_(X2_SENSORLESS, if (!(index & 1)) stepperX2.homing_threshold(value));
+            TERN_(X2_SENSORLESS, if (!index || index == 2) stepperX2.homing_threshold(value));
             break;
         #endif
         #if Y_SENSORLESS
           case Y_AXIS:
             if (index < 2) stepperY.homing_threshold(value);
-            TERN_(Y2_SENSORLESS, if (!(index & 1)) stepperY2.homing_threshold(value));
+            TERN_(Y2_SENSORLESS, if (!index || index == 2) stepperY2.homing_threshold(value));
             break;
         #endif
         #if Z_SENSORLESS

Marlin/src/inc/SanityCheck.h

   );
 #endif
 
+#define COUNT_SENSORLESS COUNT_ENABLED(Z_SENSORLESS, Z2_SENSORLESS, Z3_SENSORLESS, Z4_SENSORLESS)
+#if COUNT_SENSORLESS && COUNT_SENSORLESS != NUM_Z_STEPPERS
+  #error "All Z steppers must have *_STALL_SENSITIVITY defined to use Z sensorless homing."
+#endif
+#undef COUNT_SENSORLESS
+
 #ifdef SENSORLESS_BACKOFF_MM
   constexpr float sbm[] = SENSORLESS_BACKOFF_MM;
   static_assert(COUNT(sbm) == NUM_AXES, "SENSORLESS_BACKOFF_MM must have " _NUM_AXES_STR "elements (and no others).");