This is an example USDA result from creating an Xform and adding two Payloads to it. The first payload target prim in this case is in the file C:/path/to/file.usd with the prim path /World/some/target and the second is the default prim in the file C:/path/to/other/file.usd.

#usda 1.0
(
    defaultPrim = "World"
)

def Xform "World"
{
    def Xform "ref_prim" (
        prepend payload = [
            @C:/path/to/file.usd@</World/some/target>,
            @C:/path/to/other/file.usd@
        ]
    )
    {
    }
}