From MEPIS Documentation Wiki
For example if on MBR you have GRUB and you want that GRUB to load another GRUB that's installed in the root of hda1 partition then you have to edit /boot/grub/menu.lst for the MBR GRUB and add these lines:
title MEPIS 8 at sda1 rootnoverify (hd0,0) makeactive chainloader +1
The makeactive command is limited to primary partitions so (hd0,3) which is extended or greater will return invalid device to resolve try
title MEPIS 8.5 at sda6 rootnoverify (hd0,5) #makeactive chainloader +1
If you are attempting to chainload to a system that uses GRUB2, the method above does not work. If that is the case, try doing something like what's shown below to chainload from the Mepis grub legacy to a partition that has grub2 as its bootloader.
title Chainload to sdb7 with grub2 on it's root kernel (hd1,6)/boot/grub/core.img