That could be very well possible. i only know what i know. and i don't what i don't. And in this case, i simply know it works. Just not the how.Since the TPU chips are tiny, looking at the picture of the dual-TPU M.2 card I suspect the larger chips are some sort of PCIe bridge or switch chips. That would allow the card to split the lanes itself, without help from the host system. I can't imagine it would be possible to 'auto-sense' the need for bifurcation and then split the lanes on the host side.
And if needed, there's always makersfab: GitHub - magic-blue-smoke/Dual-Edge-TPU-Adapter: Dual Edge TPU Adapter to use it on a system with single PCIe port on m.2 A/B/E/M slot