The wavelength calibration process may be performed using either a single arc frame, or two frames between which the wavelength scale is interpolated.
The advantage of using a single frame (possibly generated by co-adding 'before' and 'after' arc frames) is that this allows the resolution of the final spectrum to be easily investigated.
This is because the arc spectrum is always extracted in an identical manner to that of the object; using the first (and in this case only) supplied arc frame.
Using two frames and having to interpolate the wavelength scale will be most useful when there is a significant shift during the exposure such that co-adding the two arcs would produce a frame with 'doubled' lines.
It should be noted that in the two-arc situation the generated wavelength scale is only an estimate based on the assumption that difference between the two arcs was due only to a stable linear variation between the exposures.
Martin Clayton