If two massive, 40-mile long, 40-foot-diameter tunnels that would direct Sacramento River water around the fragile Sacramento-San Joaquin River Delta to Central and Southern California are too big, too expensive and too scary to contemplate, how about splitting the difference and going with a single tunnel? That’s been the response of some officials and observers after actions by a number of the water agencies that were slated to participate in the $17-billion California WaterFix put the fate of the twin tunnels in doubt.