Welcome to the ROG forum.
The i7-4790 has 16 pcie lanes.
The M.2 slot supports pcie mode and uses x2 pcie lanes from the cpu. With two Nvidia gpu's running at x8 x8, there is nothing left for the M.2 slot.
Nvidia gpu's need x8 pcie lanes to work, if you had AMD gpu's this could be possible as AMD gpu's need only x4 pcie lanes to work.
I run sli myself and I like the extra performance in games that support sli.
My suggestion would be to keep sli and use a sata ssd.