Rearrange Disk Partitions

Start computer with gparted LiveCD and start GParted. The data partition is "/dev/sda1".

Use "Resize/Move" to move /dev/sda1 partition to the end of the disk, before the swap partition. Shrink 928GB to 295GB, where 87GB are used. It took about 5 hours. You can only shrink the partition without moving them to the end, which will be faster than the previous "shrink + move" method.
Right click /dev/sda1 -> Manage Flags, uncheck the "boot" flag.
Create a new 50GB partition /dev/sda3 with EXT4 format, label it as "system1" and quit GParted;
Start computer with new system (Mint 17 64bit here) LiveCD;
Install Mint, select "Something else" when asked for "Installation type";
Select /dev/sda3, click "change", then choose "Ext4 journaling file system" in "Use as", then choose "/" as the "Mount point", set /dev/sda1 mounted at /home and the old swap partition as swap;
Install the system, then reboot;

Transfer old data

The old partition is mounted on /home, so the old home folder is now /home/home/chad. So we need:

Move sub-folders of /home/home/chad to /home/chad;
After some days when it's sure all data have been transfered, delete all folders and files under /home except "chad" (new $HOME folder);

Now on /dev/sda3, there is only one folder "chad". When install the second system, just mount /dev/sda3 to /home, everything will be ok.

Start "Terminal" on LiveCD.

Detect disk: gdisk /dev/sda, notice your detection target is the whole disk instead of a partition, so "sda" can't be written as "sda1", or there will no MBR be found.
Backup MBR: sfdisk -d /dev/sda > parts.txt, you can recover MBR with sfdisk /dev/sda < backup-sda.sfdisk , or with "dd" command:

Backup: # dd if=/dev/sda of=/backup/mbr_sda.bak bs=512 count=1 Restore: # dd if=/backup/mbr_sda.bak of=/dev/hda bs=512 count=1

Backup MBR and grub.cfg:
Install centOS 6.5 Minimal with USB stick;
Choose "Create Custom Layout", create a new partititon, mounted at /, and mount sdb1 as /home;
Uncheck "Install boot loader on /dev/sdb";
Reboot system. There is no change in boot menu, startup origin system (Mint 17), run update-grub with root.
Reboot, you can see the CentOS is in boot menu.