The actual gauge is not dual. They are individual gauges and can be used alone or in tandem.
However, they must be operated by a DUAL output sensor to operate correctly as a dual setup. If a single unit sensor is used for a dual setup they will misread.
Look at the gauges carefully. Are there any decals or identifying labels?
Look at the back also for the mfgrs. name.
Have you contacted American Diesel. THey are the gurus for the Ford Lehman engine.
They may be able to help. But they are going to ask about the labels/mfgrs. of the gauge.