Can't install Mountain Lion on a machine that has Mavericks installed, even on a separate disk/partition
The only thing you can do is install ML on an external drive on another mac running 10.8.x then copy the partition using SuperDuper on your 10.9 mac

Let's say your mac is A, 10.8 mac is B and usb HDD is C :


Install Mac Os 10.8 on C using B computer
Create a partition on A (with ± 50GB)
Duplicate C on the A new partition using SuperDuper


And voilà !