I don't have an answer yet. The one screensaver I installed (a) only blanked out the ChromeOS browser window and not the displays (b) did not actually power off the screen (c) did not do anything to the second display. Maybe what I'm really looking for is a power saver mode.
Screensavers no longer serve the function they once designed for. Back in the day of phosphorous and cathode ray tubes and even early model plasma and LCD displays it was necessary to prevent pixel burn-in. Today's displays are much more durable, however, most modern screensavers require that the machine run at full speed to render exotic images. Furthermore while LCD light sources are lasting longer than ever before we still tend to burn them out and discard them. All but the most expensive monitors are not repairable.
** When I worked for IBM they did not want us to power off our computers at night but management insisted that we power off the monitors. Presumably the electricity costs for the monitor was significant and the CPU boot time impacted productivity.