A "pair of organs" (MMario, above) was what we call today an organ. I don't know why: maybe it referred to the keyboard and pipes as separate parts, or maybe it was just one of those strange idioms like a "pair of stairs". A "modern" organ may have the keyboard a long way from the sound-producing part, but I'd have expected a mediaeval organ to have them pretty close together, if not integral (as they often are today).

Steve