I spent some time searching the web about Remote Desktop, fullscreen and multiple monitors, so I decided to write down my findings to avoid having to search for them again.
/span for multiple monitors
If you pass /span to mstsc.exe, the target session’s desktop will become a huge rectangle that equals to the summary area of your physical monitors. This way the remote desktop window will fill all of your screens. The downside of this approach is that both screens are part of one desktop on the remote machine, so if you maximize a window there, it will span all of your monitors. Also, a dialog that is centered, will show up right on the border between your monitors. There is software on the web to workaround that but I’m fine with keeping my windows restored and sizing them myself. Also Tile Vertically works just fine in this case.
Saving the /span option in the .rdp file
There is a hidden option that isn’t mentioned in the description of the .rdp format:
Just add it at the bottom of the file.
Saving the /f (fullscreen) option in the .rdp file
screen mode id:i:2
(By default it’s screen mode id:i:1, which is windowed).