The question is a bit strange: Every Logitech wireless keyboard/mouse comes with its own receiver. If both keyboard and mouse are supplied with the Unifying receiver, you can pair both devices with one of the two receivers (regardless of which one) and thus only have one USB port occupied.
Buying an additional Unifying receiver only makes sense if the receiver has been lost or (with Logitech keyboards/mice that support more than one device) an additional machine is to be operated with the same keyboard/mouse. If necessary, a Unifying receiver can be used for this purpose, which has been freed up by pairing two devices to one receiver, as described above.
If the keyboard/mouse are from different manufacturers or are not Unifying-capable, they cannot be paired with a Unifying receiver.