I am running SW 2015 SP5 and I have the same issue here. Never used it before, though.
Are your dual monitors identical in term of resolutions?
This is an interesting idea. However, I am not sure if it is technically feasible/allowed by Microsoft. I have not seen any applications allow full screen across multiple monitors. Microsoft Excel, Word, and Internet Explorer don't do it nor do Chrome or Paintshop Pro. It is similar to Microsoft not having "Maximize" across multiple monitors. SOLIDWORKS spanning displays is custom coding that resizes a non-maximized window to take up multiple screens, mimicking maximizing across them.
One thing you may want to investigate is if you video card driver and related software has anything that may help. At the hardware/driver level, it may have options to combine two monitors into acting like one and then standard things like maximize and full screen would likely work across them. This is similar to how you may see at tradeshows or massive sports displays where multiple monitors are combined to display one image as if they are one.
I hope this helps,