If you wish to format your computer and you have two os installed of the same company but two different versions e.g. one is xp and the other is vista and you just have the cd of the lower version then insert the cd of the os you have at the booting time i.e. after you start your computer.
As per my suggestion,you should not format two OS of same company like windows or linux.If you have two OS of different company then you easily make the back and then after formatting.
I too agree with you, need not to format two OS of the same company.If you format two OS of different company then then you have two choices of different companies and then you can format.
I want to say that There is symbiont relationship between the two OS. So We can not format two OS of same company. So We need to upgrade two OS of different company as well and also format it as soon as far.
Yes you can install it on different drives like you can install XP on C drive and vista on D drive of your hard disk