Added HEATERS_PARALLEL (Request from reifsnyderb)

This allows a hot end with two heaters and a FET for each heater. This is useful if the FET is not capable of heating two heaters.

Marlin/Configuration_adv.h

   #define PS_ON_ASLEEP LOW
 #endif
 
+// Control heater 0 and heater 1 in parallel.
+#define HEATERS_PARALLEL
+
 //===========================================================================
 //=============================Buffers           ============================
 //===========================================================================
   #error "You cannot use TEMP_SENSOR_1_AS_REDUNDANT if EXTRUDERS > 1"
 #endif
 
+#if EXTRUDERS > 1 && defined HEATERS_PARALLEL
+  #error "You cannot use HEATERS_PARALLEL if EXTRUDERS > 1"
+#endif
+
 #if TEMP_SENSOR_0 > 0
   #define THERMISTORHEATER_0 TEMP_SENSOR_0
   #define HEATER_0_USES_THERMISTOR

Marlin/temperature.cpp

   static unsigned char temp_state = 0;
   static unsigned char pwm_count = (1 << SOFT_PWM_SCALE);
   static unsigned char soft_pwm_0;
-  #if EXTRUDERS > 1
+  #if (EXTRUDERS > 1) || defined(HEATERS_PARALLEL)
   static unsigned char soft_pwm_1;
   #endif
   #if EXTRUDERS > 2
   
   if(pwm_count == 0){
     soft_pwm_0 = soft_pwm[0];
-    if(soft_pwm_0 > 0) WRITE(HEATER_0_PIN,1);
+    if(soft_pwm_0 > 0) { 
+      WRITE(HEATER_0_PIN,1);
+      #ifdef HEATERS_PARALLEL
+      WRITE(HEATER_1_PIN,1);
+      #endif
+    }
     #if EXTRUDERS > 1
     soft_pwm_1 = soft_pwm[1];
     if(soft_pwm_1 > 0) WRITE(HEATER_1_PIN,1);
     if(soft_pwm_fan > 0) WRITE(FAN_PIN,1);
     #endif
   }
-  if(soft_pwm_0 <= pwm_count) WRITE(HEATER_0_PIN,0);
+  if(soft_pwm_0 <= pwm_count) { 
+      WRITE(HEATER_0_PIN,0);
+      #ifdef HEATERS_PARALLEL
+      WRITE(HEATER_1_PIN,0);
+      #endif
+    }
   #if EXTRUDERS > 1
   if(soft_pwm_1 <= pwm_count) WRITE(HEATER_1_PIN,0);
   #endif