You can install as many screen readers as you like, as long as you don't run them at the same time (something I actually do from time to time for various reasons) you shouldn't have any problems. Cobra, NVDA, jaws, and window eyes can all be installed on the same machine, and all will read just fine provided they are only run one at a time. Running multiple screen readers simultaneously does work, but sometimes causes conflicts either with keystrokes, or screen access, but it is a nice way to check that your screen reader of choice is seeing everything it should.