See
Here for how I set up Kubuntu and Fedora to dual boot.
If you want to share your data between the two distros, each distro will need it's own /home, but you can create a separate partition for data (Mount to /mnt/data, and link to /home/user/documents)
As Nylex said, install either distro and then install the other. With the first one that you install, you will need to set up the partitions.
My partition table is:
Code:
/data 100GB ext3 Data partition, common for all distros
swap 2GB swap Swap Partition, common for all distros
/ 20 GB ext3 Root partition for Kubuntu
/ 20 GB ext3 Root partition for Fedora
Free 60 GB For other distros, if I want to install some others.
--Ian