Jquery barcode plugin username and password

First you need to choose the suitable barcode for your requirement. See this:
http://www.codeproject.com/Articles/690035/Choosing-the-Right-Barcode-for-Your-Application

For generating barcodes, you can use barcode fonts which converts text to barcode symbols. There are fonts for several types of 1-D barcodes.

About changing the source code, I'm not sure why you need to do that. If you want to combine 2 values into one barcode, simple concatenate the 2 strings into one long string then encode it into a barcode.

 I found these plugins, but I can't change the source.. I tried everything but it doesn't work..

There are various jquery free barcode plugins are available on internet.

JQuery plugin : barcode -

http://barcode-coder.com/en/barcode-jquery-plugin-201.html

http://www.jqueryscript.net/other/Simple-jQuery-Based-Barcode-Generator-Barcode.html

You can use some unique combination of User Name and Password to create personal card, and same combination you can use to create barcode using jQuery Plugins.