You may recall the Razer project Valerie - the supercharged laptop with integrated 3 way monitors, looked lovely but came at serious cost - its also not actually available as far as we know.
However, LeSlide from Slidenjoy.com ran a Kickstarter back in 2015 for an add-on modular 2 screen expander with USB connection. It raised $600k so that should tell you something. The are now gearing up for a Spring release.
What is is? - LeSlide is a clip on two screen LCD expander system which uses a single cable to drive the twin additional displays (USB 3 and USB -C requires no additional power)
Once connected, you get the additional screen real estate in a nifty fold out system.
They are now taking orders starting at $100 off when you preorder for $349. Available at 13/15 and 17 inch monitors. Expected shipping Spring 2018 - for Mac and PC
The first standalone semi-modular from Pittsburgh Modular