Suppose! If you have one hard disk and you want to install an operating
system on it then your OS will only look for Primary Partition. That doesn't
matter that where the partition is? Either it is on Master or Slave, Primary
or Secondary.
If you have more than one hard disks and you want to install another OS on
your second hard disk then you must care of many things. The very first
thing: is your current operating system is supportable with new one for dual
booting.
If no, then you have to create 2 Primary Partitions for each HDD and use a
third party softwares like XOSL [Extended Operating System Loader] which can
create a menu for both operating systems and can active the Primary Partition
for the drive you will use.
But all these things need a advanced knowledge else you can make your PC
unstable.
Let us know!