Dual extruder setup in Marlin
I am not sure what the hardware config is for the Tevo Tarantula

Make sure your configuration.h file is setup for your hardware.
The extruder defines are describe in Conditional_LCD.h

It looks like the configuration.h file on GitHub is configured for a single extruder.

For example, if you have 2 hotends; but, "HOTENDS=2" is not set then the I/O will not be configured for the 2nd hotend.  I just looked at the code and if HOTENDS == 1 then the MOSFET_D_PIN will be used to control FAN1 (which sounds very similar to what you are describing that you are seeing).

  #if HOTENDS == 1
    #define FAN1_PIN     MOSFET_D_PIN
  #else
    #define HEATER_1_PIN MOSFET_D_PIN
  #endif