Want BOTH Stable Branch and Beta Branch Insurgency clients installed separately? Here's how!
Keep Steam set to the Stable Branch and use SteamCMD to download and maintain a separate Beta Branch client install.

Version for OSX users:
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=496199639

SteamCMD is the command line version of your familiar Steam Client, download link below:

http://media.steampowered.com/installer/steamcmd.zip

Create a folder in C:\ drive and name it “SteamCMD” (It should look like this: C:\SteamCMD ), open previously downloaded ZIP file and extract “steamcmd.exe” into the “SteamCMD” folder you’ve just created.

Open your “steamcmd.exe” and let it update, now you need to login with your Steam account:
Example:
login peter354 s0m3th1ng


It will verify your account and ask you for Steam Guard code.

Open your Email which you used to create Steam Account and retype Steam Guard Code into Steam Command Line tool, hit "enter key" and wait till it verify your account (couple of seconds), once it's verified, close SteamCMD tool and proceed to next step.

Now create a text file in “C:\SteamCMD” and name it for example: ins_beta (It should look like this:C:\SteamCMD\ins_beta.txt )

Open said text file and copy/paste there commands shown below:
Now edit variables show below to correspond with your actual data:

“your_login” - Login name of your Steam Account (example: peter354 )
“your_password” - Steam Account password (example: s0m3th1ng )
“your_path” - exact path you want to install your Beta client to, example: E:\Games\Ins_beta, keep this file path without any special markings or language-specific characters !!

Example file with exact data:
Once you’ve edited those, save/exit the file.

Create shortcut of “steamcmd.exe” into your desktop (or anywhere you like it) and add this parameter to the “target” text field:

It should look like this: C:\SteamCMD\steamcmd.exe +runscript Ins_beta.txt

!!!! Always log out (or shut down) from Steam Client before you launch this script !!!!

Now launch the shortcut and let the game download, same shortcut will “update” your game if new update come out (just launch it again).

Once the game is downloaded and SteamCMD quitted, you can launch Steam Client.

Launching game directly:
Once the game is downloaded, open folder you've specified in SteamCMD script and launch game directly from it's executable file (insurgency.exe)

Launching game via Steam Client:
Go to Steam Client Library, Add Game, Add non-Steam Game, find "insurgency.exe" from the folder you've specified in SteamCMD, mark it, click on Add Selected Programs, done.

You can get one of these errors:

Error code 5: Wrong password entered, may appear also if you have unsupported character within the password,

Error code 65: Anti-spam timer if you spamming login, wait 30+ minutes and try again

error! app 222880 state is 0x1 after update job:
Unsupported character in <path> (for example: ?, !, etc. and probably some language-specific characters as well) or invalid path.

"Failed to load script file..."
Specified script file does not exist, double-check spelling and file extensions (Windows hides file extensions by default so be careful to not have ".txt.txt" file).

ATTENTION
SteamCMD will download all subscribed items for Insurgency, including maps.