We have a piece of property with two separate homes (buildings) on it. Is this referred to as a duplex or something else? Most definitions for a duplex appear to assume that they share a common wall or roof in which our two buildings are separated by at least 20ft. Sorry if this is the wrong stack exchange, please advise if so.
It's "a property with two buildings on it".
You are right that a duplex is typically considered a single building partitioned into two living spaces, often multifamily.
I don't know of any widespread terms for setups with a single property with two similarly sized or even the same building built twice, as I believe that situation is rather rare. However, when one building is smaller, there are a variety of terms for the situation as it is abundant, especially prior to the days of onerous Single Family Zoning.
Commonly, when you have a main house, then a smaller house, the smaller house is called an Accessory Dwelling Unit, or ADU. ADUs have tons of different regional names, granny flats, carriage houses, guest houses, mother-in-law suite, in-law apartment (even though that implies it's built into the main house but some people call them that anyway even if another building), and others. As the names implies, these are often small buildings, that family live in, or that are rented out for extra income.
ADU is the term for the building, not the piece of property.
Additionally, outbuilding is often the term for separate buildings such as garages or sheds. While usually most garages and sheds are spartan buildings, some are pretty elaborate, and usually what makes a building an outbuilding rather than "a building" is the lack of habitable space.
Sometimes, it all gets blurred, as recently people have begun building ADUs which consist of a garage on the ground floor, with basically a studio apartment above the garage, as a way to squeeze extra habitable space on desirable city lots.
I believe it is just two houses built on a single plot, which can be identified as two single houses if owned by separate owners - each will then have a deed/title that lists the floor area of the house and the size of the land the house occupies. These pieces of information are required for building permits and tax purposes.
You can put more than one house on a single plot if allowed by the local zoning rules/ordinance. I am not aware of the name for it but may be called a multi-house property. However, it is a headache for the property owner when selling and re-financing. See this article https://www.godownsize.com/multiple-houses-one-lot/