When cleaning up older MacOS X computers, I still find myself installing MacOS X 10.8 in special situations. However, most days it’s 10.9 as the normal MacOS X version. To make my life easier I start off with a 16GB USB stick. In order to create a Dual Boot MacOS X Install USB Stick, I  partition the USB stick into two 8GB halves. I like to use the MacOS X Installer for Yosemite (10.10) or Mavericks (10.9)  installer to make the USB stick.

For MacOS X 10.10, start by using the Terminal and doing:

sudo /pathToInstaller/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia  --volume /Volumes/partitionToEraseOnUSB --applicationpath  /pathToInstaller/Install\ OS\ X\ Yosemite.app --nointeraction

For MacOS X 10.9, start by using the Terminal and doing:

sudo /pathToInstaller/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia  --volume /Volumes/partitionToEraseOnUSB --applicationpath  /pathToInstaller/Install\ OS\ X\ Mavericks.app --nointeraction

For the MacOS X 10.8 USB partition, follow this step by step guide at Macworld.