The Asus X99-Deluxe II seems to come with everything you need to add 2 M.2 drives to it.
They are based on older tech though (especially compared to Z270) when m.2 drives were not as popular yet, the new x299 boards have more but that does not really help you unfortunatelly
You must've skipped over asrock, they put out some X99 refreshes a year? ago with dual m2 wired directly to the cpu as the bandwidth gods intended; tachi and professional.
Identical board in white (I cannot find an actual physical chip or port difference, perhaps firmware? on the audio is different but literally nothing else): http://www.asrock.com/mb/Intel/X99 Taichi/
Both m2 are wired to cpu x4 lanes, muxed with sata also if you like putting regular in your vette or whatever.