Create a partition temporarily named /home-new, and then copy the contents of /home to it.
I think the command to use would be "sudo cp -dpR /home/* /home-new/". If you have links, check if they are OK.
When you share a home directory, you can have problems with the ~/.kde/ or ~/.gnome2/ settings being changed. You can still share the /home partition. Use a different home directory for each partition. You don't even need to use a different username. You can change the location of your home directory when creating a user, or later using the "users & groups" config program of your distro, or directly by editing the home directory field in /etc/passwd.
One thing to look out for, both ways is that the range of UIDs for regular users can differ between distro's. Mandrake & Red Hat start at 500. SuSE starts at 1000.
You can change this behavior in the /etc/login.defs file:
This is from SuSE 10.2
# Min/max values for automatic uid selection in useradd
# SYSTEM_UID_MIN to SYSTEM_UID_MAX inclusive is the range for
# UIDs for dynamically allocated administrative and system accounts.
# UID_MIN to UID_MAX inclusive is the range of UIDs of dynamically
# allocated user accounts.
# Min/max values for automatic gid selection in groupadd
# SYSTEM_GID_MIN to SYSTEM_GID_MAX inclusive is the range for
# GIDs for dynamically allocated administrative and system groups.
# GID_MIN to GID_MAX inclusive is the range of GIDs of dynamically
# allocated groups.
You might want to change your UID & GID to a number that is in range of both distro's if they differ. Make sure your UID and GID numbers match if you want to use a common data partition.
You would want to do the same thing if you had an external mass storage drive with an ext3 or xfs or reiserfs filesystem. They you would have access regardless of which distro you booted to.