hmm, looks like the X series might be operating differently from the XI series. From your manual:
page x, storage section: When the M.2_2 Socket 3 is operating in PCIEX4 mode, SATA port 5, 6 will be disabled (bottom slot)
So it looks like @Carlyle2020 was half correct. My apologies carlyle
If you insert that 2nd 970 EVO it should normally run in x4 by default (if it properly detects the 970's capabilities) but you will lose the bottom 2 sata ports, not 1 as he states. Looks like the Z370 chipset is different from the 390 in that regard. You will NOT lose GPU capabilities.
The PCIe slot uses CPU lanes, while the m.2 uses chipset lanes
My M11Formula doesn't have that problem. Both my 960 and 970 run at x4 speed and all sata ports are in use (infact i even had to add a sata-port pci-e addin card to connect the 4th hard drive :p)