Source Engine games will usually not let you launch more than one instance.
Launch the shortcut as many times as you need.
The -insecure will prevent you from joining any Valve Anti Cheat (VAC) secured servers.
You can adjust the width/height for your monitor.
At the end of Target, add -windowed -w 1920 -h 500 -insecure +m_rawinput 1 +sv_lan 1 +sv_allow_lobby_connect_only 0.
Right-click the new shortcut and select properties. Right-click left4dead2.exe and click Create shortcut. Open the Left 4 Dead 2 directory by right-clicking Left 4 Dead 2 in Steam -> Properties -> Local files -> Browse game files…
In options, load the Source Engine preset.
Install and run Universal Split Screen: see the quick start guide.