There are a couple of possibilities for what you might be thinking of:
* Aftermarket modifications: It is possible for a farmer to add a second fuel tank to their Farmall "M", perhaps for increased capacity or convenience. This would be a custom modification.
* Similar models: Some Farmall models, especially later ones, *did* have dual fuel tanks. Perhaps you are thinking of a different model, like the Farmall "Super M".
* Misunderstanding: The term "duel fuel" can sometimes refer to engines that can run on multiple fuels, like gasoline and kerosene. The Farmall "M" was generally a diesel tractor, so this wouldn't apply.
To be sure, you should provide more context about the specific tractor you are referring to. It would be helpful to know:
* Year of manufacture
* Specific model name (e.g., "Farmall M" or "Farmall Super M")
* Any visual clues about the tank arrangement
This will help to clarify what you are asking about.
