Handle dual gantry machines better

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

 
     // Check for commands that require the printer to be homed
     if (may_move) {
-      if (axis_unhomed_error()) gcode.home_all_axes();
       #if ENABLED(DUAL_X_CARRIAGE)
         if (active_extruder != 0) tool_change(0);
       #endif
+      if (axis_unhomed_error()) gcode.home_all_axes();
     }
 
     // Invalidate Mesh Points. This command is a little bit asymmetrical because