Install Mavericks on second partition in Yosemite-only system?
You have to cheat the Mavericks Installer by running it on a "pre-Yosemite" system.
The system version is determined by the key "ProductVersion" and the associated string in the file SystemVersion.plist. Replacing the 10.10.x string with 10.9.x or 10.8.x will do the trick.

Beware: After manipulating said file you can't open most of the current apps anymore. 

Prerequisites:  


a current backup of your Mac
already downloaded OS X Mavericks Installer
a Mac capable of running Mavericks


Age your system:


Log-out any other user and log-in with your admin account.
Quit any running app and open the folder containing the Mavericks installer.
Open Terminal.app.
Make a backup copy of SystemVersion.plist first by entering:

sudo cp /System/Library/CoreServices/SystemVersion.plist /System/Library/CoreServices/SystemVersion.plist.backup


and hit ↵
enter your admin password and hit ↵ (the password won't be visible!)
Modify SystemVersion.plist:

sudo nano /System/Library/CoreServices/SystemVersion.plist




change the both strings 10.10.1 (or 10.10.0) to 10.8.1 or 10.8.0

write the changes to disk with ctrlO followed by ↵
and exit nano with ctrlx
Recheck the successful change by entering:

cat /System/Library/CoreServices/SystemVersion.plist 

Enter exit, hit ↵ and quit Terminal.app.
Start Install OS X Mavericks and choose your already created second partition to install to. You may have to click "Show all partitions" if your second partition isn't visible initially.
After installation and configuration of Mavericks is finished revert the changes of SystemVersion.plist on the Yosemite partition:


Rejuvenate the Yosemite system:


Open Terminal.app in your Mavericks admin account and enter:

sudo rm /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist


and hit ↵
enter your admin password and hit ↵ (the password won't be visible!)

then enter

sudo mv /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist.backup /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist


and hit ↵
Enter exit, hit ↵ and quit Terminal.app.


If the install fails and you are urged to boot to Yosemite (which probably fails), try to boot to Recovery Mode or Internet Recovery Mode, revert the changes of the file SystemVersion.plist on the Yosemite volume from the command line in the same manner as described for the successful Mavericks install in step 10.
Then restart from scratch.

(Tested in a Yosemite VM...works)