The problem is that your computer has no way of knowing that there are two operating systems - unless you tell it. When you start the pc, it looks for a certain type of drive/partition to know which one contains an operating system and once it finds one, it just goes into the normal boot-up routine.
If you want to use two operating systems without having to remove one drive or disable it in BIOS, you need to make a couple of changes which will add an extra option when you start the computer - so it can ask you which operating system you want to boot from.
This article gives a pretty good walk-through of how to do it just using Windows' own tools - Set up a dual-boot system (from Windows Vista Inside Out)
- it also suggests a free download that should make it easier.
Alternatively, look at another free application - EasyBCD - Take control of your boot loader | NeoSmart Technologies
- which is widely used and has a good reputation.
Just one thing to bear in mind. Even if you are physically able to boot from either drive, you may still have problems using an installation of Windows from another pc. Firstly, Windows is not a "one-size-fits-all" installation - some hugely important files will have been installed that are specific to the other pc's hardware and that could make it unstable or even inoperative on a different computer. Secondly, if the version of Windows was "OEM" - which is almost always the case if it was pre-installed by the manufacturer or system builder - then it is effectively locked to the original pc's hardware and can not be transferred to another pc without you purchasing a new, full licence. It may work for a while but as soon as Microsoft try to verify the activation, it will fail and show up as an unlicenced copy - the same as a "fake" version would.