In GLFW, you can tell a window to go fullscreen on a particular monitor when you create it, but is there any way to make it stretch across multiple monitors?
On most platforms, what you are asking for is not really possible using a framework as portable as GLFW.
Fullscreen modes generally fill one logical display. You need something lower-level to setup multi-monitor topologies (AMD and NV have entire APIs and driver settings for this).
You can stretch a window across multiple monitors though and using the DECORATED flag (specifically turning it off), you may be able to make this window spanning multiple monitors appear to be fullscreen (e.g. no border / title bar). Hiding the taskbar (Windows) / launcher/menu (OS X) is another matter though.