How to make a bootable Yosemite Drive under El Captain (not installation drive)
You can't simply open Install OS X Yosemite.app and install Yosemite somewhere if your Mac is booted to OS X El Capitan. The installer app will obstruct this. You have to use a detour:

Download the Install OS X Yosemite.app from the App Store. The Yosemite installer must be "purchased" previously. You will find it in your purchase list then and you can re-download it. Create a bootable installer thumb drive:

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app


You may have to replace the proper path to Install OS X Yosemite.app and the name of your thumb drive in /Volumes/MyVolume in the above command.

Reboot to the bootable installer thumb drive. After starting the installation you will be asked where to install Yosemite. Choose an empty partition which mustn't contain an El Capitan operating system. You may also install to a second thumb drive (> 16 GB).