Added Y_DUAL_STEPPER_DRIVERS

Enables two stepper drivers to be used for the Y axis (useful for
Shapeoko style machines)
Each Y driver can be stepped in either the same way or in opposite
directions, accounting for different hardware setups (leadscrew vs. belt
driven)

Marlin/Configuration_adv.h

   #define EXTRUDERS 1
 #endif
 
+// Same again but for Y Axis.
+#define Y_DUAL_STEPPER_DRIVERS
+
+// Define if the two Y drives need to rotate in opposite directions
+#define INVERT_Y2_VS_Y_DIR true
+
+#ifdef Y_DUAL_STEPPER_DRIVERS
+  #undef EXTRUDERS
+  #define EXTRUDERS 1
+#endif
+
+#ifdef Z_DUAL_STEPPER_DRIVERS && Y_DUAL_STEPPER_DRIVERS
+  #error "You cannot have dual drivers for both Y and Z"
+#endif 
+
 //homing hits the endstop, then retracts by this distance, before it tries to slowly bump again:
 #define X_HOME_RETRACT_MM 5 
 #define Y_HOME_RETRACT_MM 5 

Marlin/Marlin.h

 #endif
 
 #if defined(Y_ENABLE_PIN) && Y_ENABLE_PIN > -1
-  #define  enable_y() WRITE(Y_ENABLE_PIN, Y_ENABLE_ON)
-  #define disable_y() WRITE(Y_ENABLE_PIN,!Y_ENABLE_ON)
+  #ifdef Y_DUAL_STEPPER_DRIVERS
+    #define  enable_y() { WRITE(Y_ENABLE_PIN, Y_ENABLE_ON); WRITE(Y2_ENABLE_PIN,  Y_ENABLE_ON); }
+    #define disable_y() { WRITE(Y_ENABLE_PIN,!Y_ENABLE_ON); WRITE(Y2_ENABLE_PIN, !Y_ENABLE_ON); }
+  #else
+    #define  enable_y() WRITE(Y_ENABLE_PIN, Y_ENABLE_ON)
+    #define disable_y() WRITE(Y_ENABLE_PIN,!Y_ENABLE_ON)
+  #endif
 #else
   #define enable_y() ;
   #define disable_y() ;

Marlin/stepper.cpp

     }
     if((out_bits & (1<<Y_AXIS))!=0){
       WRITE(Y_DIR_PIN, INVERT_Y_DIR);
+	  
+	  #ifdef Y_DUAL_STEPPER_DRIVERS
+	    WRITE(Y2_DIR_PIN, !(INVERT_Y_DIR == INVERT_Y2_VS_Y_DIR));
+	  #endif
+	  
       count_direction[Y_AXIS]=-1;
     }
     else{
       WRITE(Y_DIR_PIN, !INVERT_Y_DIR);
+	  
+	  #ifdef Y_DUAL_STEPPER_DRIVERS
+	    WRITE(Y2_DIR_PIN, (INVERT_Y_DIR == INVERT_Y2_VS_Y_DIR));
+	  #endif
+	  
       count_direction[Y_AXIS]=1;
     }
     
         counter_y += current_block->steps_y;
         if (counter_y > 0) {
           WRITE(Y_STEP_PIN, !INVERT_Y_STEP_PIN);
+		  
+		  #ifdef Y_DUAL_STEPPER_DRIVERS
+			WRITE(Y2_STEP_PIN, !INVERT_Y_STEP_PIN);
+		  #endif
+		  
           counter_y -= current_block->step_event_count; 
           count_position[Y_AXIS]+=count_direction[Y_AXIS]; 
           WRITE(Y_STEP_PIN, INVERT_Y_STEP_PIN);
+		  
+		  #ifdef Y_DUAL_STEPPER_DRIVERS
+			WRITE(Y2_STEP_PIN, INVERT_Y_STEP_PIN);
+		  #endif
         }
   
       counter_z += current_block->steps_z;
   #endif
   #if defined(Y_DIR_PIN) && Y_DIR_PIN > -1 
     SET_OUTPUT(Y_DIR_PIN);
+		
+	#if defined(Y_DUAL_STEPPER_DRIVERS) && defined(Y2_DIR_PIN) && (Y2_DIR_PIN > -1)
+	  SET_OUTPUT(Y2_DIR_PIN);
+	#endif
   #endif
   #if defined(Z_DIR_PIN) && Z_DIR_PIN > -1 
     SET_OUTPUT(Z_DIR_PIN);
   #if defined(Y_ENABLE_PIN) && Y_ENABLE_PIN > -1
     SET_OUTPUT(Y_ENABLE_PIN);
     if(!Y_ENABLE_ON) WRITE(Y_ENABLE_PIN,HIGH);
+	
+	#if defined(Y_DUAL_STEPPER_DRIVERS) && defined(Y2_ENABLE_PIN) && (Y2_ENABLE_PIN > -1)
+	  SET_OUTPUT(Y2_ENABLE_PIN);
+	  if(!Y_ENABLE_ON) WRITE(Y2_ENABLE_PIN,HIGH);
+	#endif
   #endif
   #if defined(Z_ENABLE_PIN) && Z_ENABLE_PIN > -1
     SET_OUTPUT(Z_ENABLE_PIN);
   #if defined(Y_STEP_PIN) && (Y_STEP_PIN > -1) 
     SET_OUTPUT(Y_STEP_PIN);
     WRITE(Y_STEP_PIN,INVERT_Y_STEP_PIN);
+    #if defined(Y_DUAL_STEPPER_DRIVERS) && defined(Y2_STEP_PIN) && (Y2_STEP_PIN > -1)
+      SET_OUTPUT(Y2_STEP_PIN);
+      WRITE(Y2_STEP_PIN,INVERT_Y_STEP_PIN);
+    #endif
     disable_y();
   #endif  
   #if defined(Z_STEP_PIN) && (Z_STEP_PIN > -1)