the best way to achieve this is to use a boot manager to select which one to boot up
My favour is bootmagic which come with partition magic (you need to run PM to parpare HDD first). PM with BM really do magic. click here
Other options are System Commander click here or OS Selector click here, all do great job.
I am sure there are methods which do not need any third party software, but get a proper software will save you a lot time