A workout step can only have a single intensity target. At the time this section of the FIT SDK was developed there were no devices that supported multiple intensity targets per step. There may be a few devices/platforms that support this now, but the majority still do not. Currently there are no plans to update this area of the SDK.
Other options include adding the target cadence as a developer field on the workout step, but the device/platform that is consuming the file will need to know to look for it. Another option is to add the cadence target to the workout step's note field. Many devices will display the note to the user during the workout, so the user may see it while performing the workout.