Ok, this is actually fairly simple to do.
I'm assuming you have the install disk for both 10.2 and 10.3.
Start by installing one of the OS's, probably best to start w/10.2. Make sure you install it on a partition, not the whole drive. Size of partitions is your call. I would recommend 4 20GB partitions, 2 for OS and 2 for data.
Once you have that OS installed. Reboot from the other OS install disk and install to a different partition.
Once you have both OS's installed you can boot from one into the other via the System - Boot disk option. Or you can hold down the Option key while booting up to select the disk to boot from (not real sure on this, and I'm writing from work on a PC, check Apple website or Help on OS X)
Hope this helps.
Wags
Wags
"Everything you have read is a lie, including this.