I don't have your board but the way I read your manual your DIMM.2 shares lanes with your first pcie x16 slot off your CPU. You have a 16 lane CPU, so the math would suggest yes you are x8 on your GPU in that slot if you use both DIMM.2 drives, in fact probably the same result if you only use one of them since you still won't have 16 remaining. You have 24 total lanes off the PCH (some of which are used by default for other resources) so you might want to use the midboard M.2 sockets instead, which share those particular lanes. There may be other limitations in that configuration but I don't find the users manual specific enough to determine it.