@deckingman okay, thanks. That's very insightful. Yes, I did some google-fu, on how to add a second stepper for the Z-axis, and the answer was to simply add a second extruder to the configurator, then re-map it in the config file, which is probably how I ended up with double values for two extruders.
As others have pointed out, that's not the way to add a second drive to an axis. Here is some info which might help your understanding.
Any axis can have a number of drives assigned to it but things like acceleration, steps per mm, instantaneous speed change, motor currents etc are all applied on a per axis basis. That is to say, any drives associated with an axis will all be assigned the same values. If you think about, that's the way it has to be because you can't have two motors driving the same axis using different acceleration values for example - they have to synchronised. For that reason, we don't use colon separators for individual drives on the same axis - every drive will have the same values by default.
Extruders are treated similarly to axes but each extruder can only have one drive assigned to it. Tools can have one or more extruders assigned to them but that's another matter. For machines which have multiple extruders, we have to stipulate the acceleration, jerk, steps per mm etc for each extruder, just as we would for separate axes (X,Y and Z might use the same values but they might equally use different values depending on pulley size or lead screw lead). We do that by using multiple values separated by a colon. It's just a shorthand way of saying the first value applies to extruder 0, the second the extruder 1, the third to extruder 2 etc.