I get what you want to have and also why, Simon.
The switch part actually means a logical switch, not a physical one.
These chips route the signal wires from the mainboard slot to both devices and coordinate the traffic from device<->device and device<->board.
Think of the NF200 switch or Lucid Hydra switch or the PLX PCI-E series.
Some companies offer chips like these within a FBGA case to use on PCBs (you need to solder them on the board) or with a closed SMD case (the ones with tiny legs) to solder them directly to wires or PCBs.
I know that you can get premanufactured constructions with these chips in various configurations like:
16x in <-> 8x & 8x out
16x in <-> 8x & 4x & 4*1x out
and so on.
You need a device that controlls data&command flow on the slot, because the devices need to alternately access the key part of the slot.
This cannot be done in a strictly alternating way (device1 - device2 - device1 - device2 - device1 - ...) but according the the PCI-E protocol, that handles requests dynamically.
That is why you need such a PCI-E switch chip, that works like a network switch.
