You could do that, but I don't really see the point, the 7870 can handle two monitors on its own easily enough. Only reason I can see for doing that is if you already have the 5450, and the only way to hook up your second monitor to your 7870 is with an adapter and you are looking to save a bit of money. If you don't already have the second card and you are having this problem, I'd say just buy the required adapter and hook up both monitors to your 7870.