The main Marlin branch is still written with the assumption that both of your extruders are identical. So all you need to do is set EXTRUDERS to 2, set your EXTRUDER_OFFSET_X and EXTRUDER_OFFSET_Y, and that's all. If you add an extra value to DEFAULT_AXIS_STEPS_PER_UNIT Marlin will just ignore it.
Your slicer should operate independently of the machine. You just have to configure your slicer's settings and tell it you want it to slice for a 2 extruder setup.
Your slicer should operate independently of the machine. You just have to configure your slicer's settings and tell it you want it to slice for a 2 extruder setup.