Answer
No, while the ArcIMSParam.js file can be edited to make a second scale bar visible and to set the units of the first scale bar, the units of the second scale bar cannot be modified. The units of the second scale bar are dependent on the units of the first scale bar.
This dependency, which cannot be changed, works as follows:
If the units of the first scale bar are set to Miles, then the viewer sets the units of the second scale bar to Kilometers. If the first is set to Kilometers, then the second is set to Miles. If the first is Feet, then the second is Meters. If the first is Meters, then the second is Feet.