Arbalètes à deux pieds frequently appear in medieval inventories from the 13th century on, often contrasted with the arbalète à une pied. The one foot variety is presumably the familiar sort spanned from a standing position with one foot in a stirrup at the front of the stock. Some have suggested that the two foot crossbows are spanned with a wider stirrup for two feet. There are two objects to this theory. First the biomechanics are very poor. Second, there is no surviving bow that looks like that, and no examples in medieval iconography.
Robert MacPherson has suggested a far more plausible theory: that they were spanned while seated on the ground with a belt hook on the string and both feet on the prod, as though performing a short leg press. That makes a lot more sense.