Boot OS X install from one Mac on another Mac
Try this:


Install a fresh copy of OS X 10.8 onto your USB disk (just download the installer from the Mac App Store and select the external disk as target drive).
Boot from the newly installed OS X.
Using Migration Assistant, transfer your data and configuration files from MacBook A's startup disk. You'll have to select the option "From a Mac, Time Machine backup, or startup disk".
Boot from the external USB drive.


I know it's not a clone, but Migration Assistant does fairly well and preserves all your data.
I did this when I had to transfer my system (OS X 10.9 on a "classic" MBP 13" late 2012) on my new SSD drive. It worked very well. I did not lose any data.

If it doesn't work, your problem may be that you're trying to boot a version of Mac OS X earlier than what came with MacBook B. Maybe installing Mavericks will do the trick.

See this article on Apple KB in order to know the original OS X version shipped with your hardware.