I don't really recommend this (since "dd" is one of those dangerous commands and you have to be really careful whenever you use "dd"), but this will tell you which linux is in control of the MBR
(Here you need to replace /dev/sda by whatever drive you are booting from. Also this is a command in a linux terminal, not in the grub command line))
sudo dd if=/dev/sda bs=1 skip=1049 count=2 | hexdump
The output will look like this
In the first case the controlling linux is (hd0,5)
ff means the controlling linux is on the boot drive, "ff 0n" means (hd0,n).
In the second case the controlling linux is (hd2,5) "8m 0n" means (hdm,n)