Plus it's unusual for a procedure to incorporate both iridium and cobalt...seperate procedures for radiation source are required by all known codes. Not withstanding the exposure time difference, the quality of the radiation will have a marked effect on contrast, latitude, sensitivity, quality, etc. of the finished radiograph.
Iridium, Cobalt, X-Ray each has its strong and weak aspects for a given application. To quote Chief Engineer Montgomery Scott "you need to use the right tool for the right job".