PCI-e. Using raisers to turn 1x and 16x to two 8x
The best you could hope to do is bifurcate the x16 slot into two x8 slots. This will require support from the upstream device (CPU or PCIe switch). If it's supported, then I believe this would be implemented by changing how the configuration registers are set, which is likely done in BIOS before the machine boots so the PCIe bus is properly enumerated. So yes, it is possibly doable, but it would depend on the motherboard design and would require editing the BIOS. And you would also need to buffer the reference clock.