DOING THIS WILL DESTROY ALL THE DATA ON YOUR HARD DISK
Make sure you back up any data you need before you do this!
1. Insert your OS 9 CD and boot from it.
2. Run Disk Utility from the CD and reformat your hard drive into 2 partitions, one called OS 9 and one called OS X (but the names don't matter). Size them based on which you use more. If you have tonnes of OS X apps and do most of your work in OS X, you might want to allocate more space or vice versa. But keep in mind that the OS X system folder is a bit bigger than the Classic system (my X system folder is 900 MB, which doesn't include /etc, /var, /usr, or /bin).
3. Install Classic onto one partition and boot (from your hard drive into 9).
4. Insert your OS X CD and install OS X onto the other partition.
Now you can hold down OPTION when the computer starts, and you'll get a "Boot Menu" with all of your available boot volumes (you'll see both hard disks, and the CD drive if it's bootable), click on the one you want to boot into and voilà you'r en that system.