The term "bay" does not relate to the rope at all, it is a "section" of the swing construction, usually two "A" frame legs connected with a top beam from which the swing hangs. The rule mentioned indicates that it is forbidden to affix ropes hanging by one end (they may have some sort of leg support, at least a knot, on the other) in the same bay (=section), where swing or swings are already mounted.
It does not necessarily mean that the rope may not be affixed to the same beam: imagine a structure with a single beam supported by three "A" frames: such construction will have two bays, and ropes could be in one, while swings in the other, they will all hang off the same beam, but the construction would still meet this requirement.