bifurcation Is the keyword you’ll want to be looking for. And before you look into it any further make sure your motherboard even supports it on that slot. Bifurcation has existed since like pcie2 or something, but just like rebar hasn’t supported on consumer platforms until pretty recently.
That said I can’t actually help you because I don’t know of anything to do this. I can only imagine it doesn’t exist because there’s not enough space on one M.2 slot to fit two M.2 ssds. Traces take up space, connectors take up space, and you’d have to somehow double stack SSDs on top of each other and they already get too hot for their own good.
