There are PCIe cards for adding M.2 like the asus hyper m.2 x16. Does it all need to be on a single pcie? One of those cards on your second slot and the gpu in the first could work, seems to me like adding some no name aliexpress card between the gpu and motherboard is asking for issues.
Buildapc
yeah, i know such cards exist but the goal here is to tap those 8 lanes you loose by using x8 GPU (such as RX 6600, RX 7600, RTX 4060 etc.) in x16 slot, people could gain extra 2 m.2 ports this way, enabling compact-ish builds with lots of storage, for example i have only only one m.2 slot on my cheapo mATX board, and two additional pcie 2.0 x1 slots i have available are pretty useless storage-wise (not faster than SATA3), it would be pretty useful in my case (though i would be able to use only one additional m.2 slot because x8x8 bifurcation support)
ah my bad, I misunderstood. Yea having this would be a nice way to use those extra 8x lanes, as long as you dont plan on upgrading the card to one that needs the x16.
*coin miners do basically what you're asking all the time.
Your motherboard needs to support the bifurcation. But if it does then what you're looking for can be done really cheaply (and ghettoly) with ease.