It has multi-monitor support it just doesn't list the resolution.
http://www.skid-inc.net/stuff/wsgf/cities_surround.jpgOne person said they got it to work by setting the game to windows mode, then pressing Alt + Enter to switch to fullscreen and that selected there resolution for them (note the menu is aspect ratio locked to 16:9, ingame is not.) If that doesn't work you'll have to hex edit the configuration file.
1) Load the game, select your single monitor resolution as your resolution option.
2) Download some sort of hex editor, I use XVI32.
3) Go to %USERPROFILE%/AppData/Local/Colossal Order/Cities_Skylines
4) Open gameSettings.cgs in your hex editor.
5) Open Windows Calculator, and change it to programmer mode.
6) In Calculator, make sure Dec is selected, enter your single monitor resolution width (mine is 1680) and then click Hex.
7) In my case it'll give me a value of (6 90), add the leading 0 if the value is only 3 digits long, so (6 90) becomes (06 90). Without going into the how numbers are stored in binary, you need to swap the first pair and last pair of numbers, so I end up with (90 06).
8) Do the same with your multi-monitor resolution so mine is 5040, which is (13 B0) in hex, so swapped becomes (B0 13).
9) Lastly, in my hex editor, I go to search, then replace, tell it to search for and replace Hex strings, I tell it to find my single monitor resolution, and replace it with my multi-monitor resolution.
10) Save the file and load the game, not the menu menu is aspect ratio locked to 16:9, the game itself is not.
I tried everything up until finding and replacing since it doesn't seem to find the string, my resolution is 1920x1080 and I got 780 so I take it it should be 80 07, but it's always the same result, string not found. do you have any workarounds or maybe another method to fix it?
Also the Alt+Enter twice doesn't work for me either.
And yes I'm sure that I'm in AppData and not in the installation folder.